I once wired up a cheap (but working) PC PSU to the Amiga. The result? Constant crashes.
When connecting the 5V rail up to a scope, the amount of ripple / noise was crazy.
I'm not sure if it's because of the cheap PSU (which seemed to lack any filtering components) or poor regulation due to the Amiga drawing such little current. I think some PSUs require a reasonable load on the 12V (?) rail in order to regulate the other rails properly.
Just saying this, as it's not always as easy as one may think.
I simply tossed the PC PSU, replaced the failing components in my lightweight A1200 PSU (filter caps failed, also replaced electrolytics) and my A1200 with 50MHz 030, Indivision, RAM and CF IDE has been stable ever since.
Steve