Each type of viewfinder has it's pluses and minuses.

An EVF such as most mirrorless use can show "exposure simulation". That's a preview of what an image will look like based upon the current camera settings. As a result, you have immediate indication whether your settings are correct and are able to adjust accordingly, if needed. Assuming your settings are good, this also will brighten up the EVF if shooting in low light conditions or with manually stopped down lenses. Optical viewfnders can't do either.

However, an EVF also draws power constantly while in use. In addition, the camera's image sensor has to be continuously powered up to provide a signal for the EVF to display. It's much like using Live View with a DSLR.... And both put a heavy drain on batteries. In many mirrorless this is further compounded by the small format batteries that many of them use in an effort to keep the camera small and light. Optical viewfinders themselves draw no power at all and the camera's sensor isn't active continuously either. As a result, compared to a DSLR with an OVF, most mirrorless cameras with an EVF get far fewer shots per charge so you may need to buy and carry more spare batteries.(Of course, readout displays and AF systems seen in the viewfinder draw some power in both EVF and OVF... there's no difference in this respect.)

There used to be a noticeable lag with EVF, too... What was displayed in the EVF was slightly delayed. Manufacturers have worked hard to counteract this and newer models show little lag. However, although it's very little now, there still may be enough delay to make difficult extremely precise timing of certain types of shots that require it.

It's more than just the different types of viewfinders too.

Today most DLSRs with OVF actually have two autofocus systems. First, they use an array of sensors that you see in the viewfinder, plus there's another array of senors embedded in the image sensor itself, to function during Live View and while recording video.

Mirrorless cameras with EVF only have one AF system, the latter type that's embedded in the image sensor.

In older DSLRs and earlier mirrorless the sensor-based AF system relied upon contrast detection and was slow. It was unusable with moving subjects. The DSLRs using the separate array with phase detection AF system were much faster and better tracking subject movement.

Today with recent mirrorless and DSLRs there's now a form of image sensor-based phase detection being used with them too, which makes for much more responsive AF that's more usable with moving subjects. However, it's still not equal to what a top-of-the-line DSLR with a separate phase detection array can do.

And while many mirrorless claim fast frame rates, if you read the fine print you'll notice many specify "with AF locked" or something similar. With continuous AF such as is used for sports and other types of action photography (where you will be using fast frame rates too!), many mirrorless have to slow their frame rates considerably because their AF system cannot "keep up". Where a DSLR with an advanced AF system might be able to shoot at 10 or even 12 frames per second, many mirrorless slow to 4 to 6 fps when used similarly.

Another difference is that OVF-based AF system arrays have between 9 and about 150 individual points, plus are somewhat centered in the image area (the number and layout varies by brand and model). EVF's sensor-based arrays commonly have far more AF points... some have literally thousands... and they tend to cover a larger portion of the image area. Some are very near 100% coverage, which means you can focus on subjects much closer to the edges and corners of the image, if you wish.

Many or most EVF/sensor-based AF systems also have a feature such as "focus peaking", which gives visual indication of what's in focus, making them ideal for use with manual focus lenses. OVFs can't do this.

And, finally, OVF with a separate AF sensor array need to be calibrated to focus the image on the image sensor. This can require fine tuning with a feature such as Micro Focus Adjust on many DSLRs, with whith the user can adjust various lenses for best possible focus accuracy, but it's rarely absolutely perfect, especially with zoom lenses that have become more and more the norm. In contrast, because the mirrorless/EVF camera's AF sensors are embedded right in the image sensor itself, there's no need for this sort of calibration.

So, in summary: In some ways EVF are superior... But in some other ways OVF are still superior. Overall, if shooting sedentary subjects such as posed portraits or products or landscapes or architecture... an EVF will work just as well as an OVF... In fact, the EVF may even be a better choice in some situations. However, if shooting fast action such as sports, wildlife and similar... both for better tracking, faster frame rates and more shots per battery charge... a DSLR with an OVF may still be a better choice.

Something neither can do.... Back in the days of film I used a variety of types and formats of cameras for different purposes. Many rangefinder cameras had optical viewfinders that showed more than 100% of the image area, had a "bright line" indicating the image area, but also visible was some additional portion of the scene outside that area. This was handy for some types of photography because you could see both what you were including in the image, as well as what was slightly outside it that you were excluding. The photographer could then choose to recompose, if they wished. Instead of a viewfinder, some view cameras I used also had a ground glass focusing screen that covered a bit more than the actual image area and could be used similarly. This was a nice feature that isn't the possible with modern EVF or OVF. All I've ever seen, at best show 100% of the image area (some show a little bit less).

If you are bothered by batteries that last too long without a recharge,
EVF will solve that problem. OVF current drain is 0 mA.

Or if you've always been wishing for a time lag in your viewfinder.
OVF delay is 0 uS.

Finally, if your OVF was too sharp and contrasty, EVF will fix that too.
OVF matched the resolution and contrast of the human eye. EVF can't
even match the image sensor performance.

Remember, the screen on the OVF is maybe 1/30th to 1/50th the area of
the computer monitor you're reading this one. How well would you like
using just one tiny corner of your monitor?

But only one thing matters: EVF requies less labor to assemble, reducing
the camera manufacture's unit cost. Of course, they pocket this savings--
and will do anything to get you to switch.

Nope. It was both. It was the intersection of both.

I posted in reply to [wdross]:

"That is why OVF can still be as viable as EVF. Some
of us that really don't have PP software have to get
it right the first time because even with a RAW file
recorded there will be very limited software to
process it if any at all." IOW, aint me who mashed
together viewfinders and workflow. I simply replied
to the existing mash-up that was already there.

And yes I said that the statement made no sense.
What DO viewfinders have to do with a user's lack
of software ? That is what makes no sense. Plus I
do stand by my comment that PP software is just
as important to SLRs as is batteries and SD cards.
And unlike batteries and cards, basic PP software
is supplied free/included with the camera.

Every advanced or enthusiast camera comes with
enuf free software to do the job. Even when I've
delivered ten dozen perfect SOOC images to the
client, I know that she will then PP them for her
specific needs. It's just what's normal.

==========================

There's reason phone photography is so popular,
and using an SLR where a phone is really more
appropriate also makes no sense. And even the
makers of phones and of SLRs acknowledge that
their wonderful products deliver results that can
need some PP, as some phones and SLRs lately
have a useful degree of PP built in. Refusing to
go beyond SOOC results makes no sense at all
unless it's mainly Point and Shoot, which is the
turf of phones ... and they do an excellent job !

FWIW, I noticed that a few of my cameras offer
in-camera PP. I've never used it cuz the tiny 3"
monitor is not a very inviting working space !
Maybe I should just junk my PC, get a huge TV
and an HDMI cable, and do my PP in-camera ?
Is anyone here already doing that ? Stranger
things happen every day .....

.

The first statement is just total nonsense. Today's SLRs, and yes, even phones, produce amazingly good photographs even on their default settings. People on this forum are more than usual interested in photography, but the majority of owners of SLRs just want good photographs right out of the camera. So the camera manufacturers keep adding more and more intelligence to the camera to allow exactly that. Only a small percentage want to do PP. When they return from vacation, they want to view their pictures, not spend hours adjusting the fine details.

For me, the only time a phone is more appropriate than an SLR is when I have my phone with me and the SLR is at home. I don't own a $1,000 phone. I prefer a $100 phone and a good SLR.

It's not that I "refuse" to go beyond SOOC, it is just that I either don't see the need to, or don't have the time. I agree with wdross. My partner has a business doing event photography, where the end result is refrigerator magnets. I take the pictures, she makes the magnets, and people pick them up on the way out of the event. We typically take 200+ photographs and have to process them immediately. We don't have time for PP, they have to be right. You may say that a 2"x3" magnet is not going to show defects, but people are picky. And the picture stays right there on their refrigerator at home or at work, so it gets seen a lot, unlike so many photographs which have extensive post processing and then nobody looks at them again.

As I said in my first post, there is room for differing opinions. Nikon v. Canon, EVF v. OVF, B&W v. color, raw v. JPEG, prime lens v. zoom, UV filter v. no filter, and on and on. My point is that anyone who gets both dogmatic and hostile about their point of view is not contributing to an intelligent discussion, which is the point of this forum.