The biggest problem was that Amiga did not have an MMU until very late, and the OS has been designed for unprotected shared memory space. It was crashy, with fragmenting leaky memory, and it could not support fork(). Later AmigaOS 4 and MorphOS struggled to add full process isolation.
In retrospect, Microsoft has been prescient in adding virtual memory to 9x, and incredibly successful in switching to the NT kernel. AmigaOS would have needed the same to survive, instead of just sitting on their multitasking-a-decade-before-Windows laurels.
Did you mean protected memory? Didn't OS/2 have both earlier?
One day I'll rip and upload a video I made in the late 80s of when I was typing up a chemistry lab report. My Amiga was producing so many guru meditation errors I had my VCR record my typing (in real-time) so I had proof that I did the report. I handed in a draft of the report and a VHS tape.
Before 1.2, AmigaOS was very unstable.