Also, PS2 mouses and keyboards have physically identical but electrically incompatible connectors, which is like a precursor of the nightmare that is USB. ADB is positively magic in comparison.
Note: keyboards and mice were pretty low risk. High risk were PS2-based barcode scanners.
Not to say there weren't problems. I had a Compaq couldn't speak to most PS/2 keyboards, so when my keyboard broke, I had to go find another Compaq keyboard on campus before I could use the computer again.