There's theoretical instability and actual instability. Yes no MP --> increase risk of a bad program bringing the whole system. But don't use cracked software/ beta software/ software written by someone who got Blitz Basic for Christmas, and suddenly Amiga becomes a very stable machine.
I like to point to all the stuff on aminet that has been created by ordinary users on aminet: if everyone was losing data all the time, how could Aminet at one stage end being the world's biggest repository of PD software?
Personally, my A4000 with 68060 and CV64 running a reasonably-heavily patched 3.1 was rock solid with the Apps I ran, most of which were original and commercial. Amazing given that the software support for the CPU itself and the RTG graphics card were basically third party kludges. And thats the thing here: after Commodire went under so much of the OS was updated by so many different people, adding CD support, RT audio and graphics, datatypes, tcp stacks, floating point libraries, screen drawing routines, menu hacks, things which really go to the heart of what the OS should be doing was all written by third parties without any central quality control.