Instead of the word "flat", my preference is the expression "wishy-washy", which of course explains everything .

As is usually the case it's a question of degree. For some people the whole point of moving to B&W is to bring out the visual drama in a scene. The main way to do that is to add contrast, and for some people it doesn't take much contrast, but for others it needs to be ramped up considerably. There's no right and wrong, but at the same time the words "suitable" and "appropriate" and "tasteful" don't lose their meaning in that scenario.

With just about any adjustment you can think of there will be a point beyond which the effect starts to look excessive for one reason or another, and overcooking is one of the most common mistakes in PP, especially for beginners. Some might argue that it's always a matter of personal preference, but if someone's preferences leave them in a group of 1 they need to consider if perhaps their preferences (i.e. tastes) could do with a bit of refining. Yes I know - it is all relative and it is a matter of taste, but if someone's adjustments look more like mistakes than improvements it's time for them to do a re-assessment of what they want to achieve.

Pushing adjustments is something that can be done for effect, but it's only in specific instances that we want specific effects. What we are discussing here is general guidelines for producing an outcome that achieves the purpose of converting to B&W in the first place.

Any beginner's tastes will be undeveloped compared to those of an afficionado and there's no shame in learning from those who have more experience and skill. The simple fact is good editing produces good results and it's possible for editing to produce results that nobody is going to consider good. I'm going to hazard a guess that nobody wants that second outcome. So maybe it's an oversimplification to say "It's all relative and therefore all outcomes are legitimate".

So are we agreed that some outcomes are desirable while some are not so desirable? So now we can consider what a desirable outcome is (and how to achieve it)....

A common result of adding contrast is that the darks can become too solid and too predominant, so you need to be able to take steps to mitigate that outcome. And the same thing applies to the highlights. One way to do that is to use the Brightness, Whites, Highlights, Shadows and Blacks sliders. However, some prefer the Curves tool. The Clarity tool can be used to add vividness but its effects can quickly become extreme and difficult or impossible to mitigate, so it needs to be used with a light hand, and typically towards the end of the workflow. Sharpening can also be used to increase vividness, but it too can be overdone, and it needs to be done alongside denoise.

Where contrast is concerned, the luminosity spectrum goes from pure black to pure white with all shades of grey in between. That spectrum covers all of the tonal possibilities so we have to work within its limitations. One implication of that is that there's always a limit to how much contrast can be implemented in a B&W conversion, so sometimes we have to choose where on the luminosity spectrum to put the most contrast. Adding contrast basically means stretching the distribution of darks and brights, and since the spectrum is limited, there's always a limit to how much the tonal distribution can be stretched. What we can do is stretch specific parts of the spectrum, which will result in an increase in contrast in that particular part of the spectrum.

Put more simply, once the tonal distribution has been stretched to its limits you can add extra contrast in the darks or the mid-tones or the brights (but at the expense of contrast in the other parts of the spectrum). Where you place the extra contrast on the spectrum is the PP editor's choice, as is the choice of location for the extra contrast. That is part of the creative expression that a PP editor has at their disposal. To that we can add the choice of overall brightness and the overall distribution of brightness/darkness.