Oh it wasn't robust by todays standards (or any standards really), but it would be wrong to infer from that that the quality of OS9 was inferior to OS8, OS7, rotting, etc.
I say as a Mac user, while not inferior to other MacOS’s, it was however inferior to the stability of the counterpart versions of Windows, more-or-less since inception. The stability gap got wider and wider as the years went on and Windows 2k made OS9 look like a joke. OS X closed that gap and then quickly surpassed it.
I still think the distinction is (a bit) valid. You could run a stressed mail server on os9 for a year with no downtime/ no maintenance. The chances that the afterthought mac port of the windows software that you need to run would be flakey? - pretty high. These apps crashed just as much on osx, at least until macs become popular enough to not be able to get away with it. The process isolation made it more bearable, but you'd still lose your work. The benefit of osx wasn't spontaneous reliability (although, some things - font handling, printing - ok, it kinda was), it was that you could now run high quality open source alternatives (it's weird to remember just how difficult that was on OS9).
