"It depends".
What you don't want to do is to have the application and/or the operating system or some other application trying to access the same drive at the same time, even with a fast SSD. It's worse on spinning media, of course, but the principle is the same with an SSD and there is a a potential performance hit if you have access clash. The point of a dedicated drive is two fold - make it fast (and often it was smaller so that the speed/price combination wasn't outrageous) and make it exclusive to avoid access clash.
Even with a fast machine and SSDs, if the OS decides to swap out to drive from memory or is reading or writing data for whatever reason (the file itself, the catalogue, and so on, depending on which application you're talking about) then you can have a performance hit.
Of course, the real world impact of that performance hit is really the important question. If it's not affecting your ability to do what you want to do, then that's all that really matters unless you're trying to benchmark.
Personally, I have my catalogue on a very fast M.2 drive which also serves as scratch disk if I'm using PS (I'm not going to be doing disk intensive things with both applications at once), and this works really well, but I doubt very much that using the OS fast SSD would seriously degrade performance in any real-world way.