I've never tried that, I've got a UEFI bios and what not, so it's not a huge deal in the bios but I can't do certain tasks, such as (press enter etc).
The main problem would occur when starting up and I couldn't select my O/S or enter safe mode, etc, etc.
But now, the problem is that if I want to keep using PS/2 then I have to suffer with double typing keys, I'm going to type a basic paragraph and paste it in here on my keyboard, (cherry reds btw), the first line will be with the USB and then the second line will be with PS/2 and you'll see the vast difference.