I think it depends how good you threat your modules.
If you have insuffiecent power in your rack it may occur that you grill a module.
If you buy used modules on MG, which gone through a couple of hands, there is always a chance that the people did some crap with it. + that they are not honest and sell you a troubled module.
QU-Bit for instance gives a lifelong guarantee. They will help always with issues and repairs for free, even if the regular guarantee time is over. I guess there are more manufacturers who do it too.
Since today, I had no function issue with my modules, only knobs or nuts got a bit loose.
But in 3 cases, MG-sellers did withold that they sell me a broken or troubled module, which I didnt see immedately. I guess the previous owners treathed them harsh.
The first module worked at the first runs, but after a short while it was not completly working - 1/3 of the features and inputs didnt response
The second module, had the problem that the nut of the trigger-input was loose and fall completly off.
The third Module had a broken clock input, the rest was working.
Things like that are pain in the *****.
Greetings
Chris