If I had to say one particular thing, it would be misuse by owners not fully understanding their amps. I personally service, modify and repair over 300 amps a year of all makes and models. I have never seen any other brand where there is more damage done by user error than the MTS amps. Everything from improper bias, instrument cables used as speaker cables, pulling half the power tubes and then running the OT at an unsafe mismatch and much more. I have struggled to come up with a reason why, but bad practices have plagued these amps for years. If I had a dollar for every time someone thought a module sounded terrible only to find out it was something related to their amp, I'd be able to buy a summer vacation for the wife and I.
There is a lot more going on inside these amps than most and I probably service more of them than anyone out there. The general quality is fine and rather consistent. I just see a lot of abuse and constantly get contacted by buyers of used amps that have issues, more-so than any of the other gear I work on daily.