The custom fps system only allows a value that can be created using two numbers with (1) no more than 3 decimal points and (2) values less than 1000. Let's take for example 29.970628 a common format in many of my videos.
29970628/1000000 You can't simplify this down to meet those two rules above.
There are many instances where I can't get the fps of my workspace to fit the video. Especially when doing recordings in OBS, I'll choose an fps like 24 and OBS will output something like 24.02145 fps...values that can't be expressed in the framework blender allows as inputs.
I also hate how blender doesn't show the total fps value correctly, but will render it correctly with all the decimal places. This is very confusing. Take for example 5/0.21, when rendered we see it does infact respect the many many decimals of this fps, but you'd never know as the inputs don't show it. So inputs don't seem to accept more than 3 decimal places but outputs will respect more than 3 places.