So it basically goes as "it depends".
A picture is better than nothing, one camera may be better than the other and so forth.

But aren't we missing a point here? ISO is a standard. Hence if a camera says it can reach such level of the standard, that camera should be good at its designated ISO.
As an example; if "XY" camera is rated 100-5000 ISO, we should safely assume that at the highest ISO rated, we still could get a relatively good image but that is not necessarily true.
Hence the question of, what is the actual working ISO, as it seems to me, these ratings they use are just selling point candy and are useless comparison in actual photography.

Like cars are rated speeds. On a speed camera, 100kph is 100kph whatever car you use. But if cars were cameras, they are rated 100kph but only happens if pushed down a hill.

Cameras are matched against each other and praised for their high ISO (performance?). But what do they really deliver? What is the actual functional or shall we say acceptable/usable ISO? My personal limit is ISO 6400. It seems beyond that, any of the cameras I've used shows varying degrees of (unacceptable) noise. It may seem trivial because that is still pretty high compared to the ASA 800 film of the olden days when we now have 6 digit ISO numbers. But do those high ISO really matters or are they just sales candy? Thoughts anyone?

Cameras are matched against each other and praised for their high ISO (performance?). But what do they really deliver? What is the actual functional or shall we say acceptable/usable ISO? My personal limit is ISO 6400. It seems beyond that, any of the cameras I've used shows varying degrees of (unacceptable) noise. It may seem trivial because that is still pretty high compared to the ASA 800 film of the olden days when we now have 6 digit ISO numbers. But do those high ISO really matters or are they just sales candy? Thoughts anyone?

The price of high ISO is noise.

Random noise represnets the loss of information from the image file.
Maybe you don't care about information, but you probably care about money.

At ISO 100, your $1800 lens looks like $1800.
At IS0 200, it looks like a $900 lens
At ISO 400, it looks like a $450 lens.
At ISO 800, it looks like a $225 lens.
At ISO 1600, it looks like a $112.50 lens.
At ISO 3200, the photo look like it was taken with a $56.25 lens...
with fingerprints on it.

The choice is yours.

There are alternatives: faster lens (e.g, a prime), good off-camera flash,
lights, or just wait for better light.

There is a way to have high ISO without noise, used in space telescopes:
cryo-cool the sensor. Short of that, there is no way to turn up the sensitivity
without getting more "static". There is always thermal (Johnson-Nyquist)
noise.

Since solid backgrounds are more common than speckled ones, clever processing
can hide th enoise by filling in tiny white spots in a dark backgroun. But is the
background really had tiny white spots, you lose. Or if there are tiny black
spots in a white background--since these are very common in real scenes.

The trouble with random noise is...it's random. It destroys information.
had tiny white speckles,

Wallen, ISO 300K is probably pretty useless but I'll bet a pic of bigfoot at that ISO would be worth a pretty penny!
Sometimes a grainy pic is better than no pic at all.
Though I prefer to shoot at ISO 100, I'll shoot at whatever it takes to get an image and it not all movement blurred.
This was shot at 25600. Is It perfect, absolutely not, and it may not meet your standards but it was good enough to be published.
I don't usually shoot this high but sometimes one has to.
This is basically SOOC, no PS or noise reduction other than in camera. Shot with a Canon 1Dx.
SS

So it basically goes as "it depends".
A picture is better than nothing, one camera may be better than the other and so forth.

But aren't we missing a point here? ISO is a standard. Hence if a camera says it can reach such level of the standard, that camera should be good at its designated ISO.
As an example; if "XY" camera is rated 100-5000 ISO, we should safely assume that at the highest ISO rated, we still could get a relatively good image but that is not necessarily true.
Hence the question of, what is the actual working ISO, as it seems to me, these ratings they use are just selling point candy and are useless comparison in actual photography.

Like cars are rated speeds. On a speed camera, 100kph is 100kph whatever car you use. But if cars were cameras, they are rated 100kph but only happens if pushed down a hill.

The price of high ISO is noise.

Random noise represnets the loss of information from the image file.
Maybe you don't care about information, but you probably care about money.

At ISO 100, your $1800 lens looks like $1800.
At IS0 200, it looks like a $900 lens
At ISO 400, it looks like a $450 lens.
At ISO 800, it looks like a $225 lens.
At ISO 1600, it looks like a $112.50 lens.
At ISO 3200, the photo look like it was taken with a $56.25 lens...
with fingerprints on it.

The choice is yours.

There are alternatives: faster lens (e.g, a prime), good off-camera flash,
lights, or just wait for better light.

There is a way to have high ISO without noise, used in space telescopes:
cryo-cool the sensor. Short of that, there is no way to turn up the sensitivity
without getting more "static". There is always thermal (Johnson-Nyquist)
noise.

Since solid backgrounds are more common than speckled ones, clever processing
can hide th enoise by filling in tiny white spots in a dark backgroun. But is the
background really had tiny white spots, you lose. Or if there are tiny black
spots in a white background--since these are very common in real scenes.

The trouble with random noise is...it's random. It destroys information.
had tiny white speckles,