I used 98se for a long time, well into the Vista era --- and indeed the lack of protection affects stability, but that also means the stability is directly related to what software you run; and indeed if you regularly used the mass quantities of sub-par "shovelware" that was prevalent in those days, it would definitely make it seem like the OS was crashing constantly, when it was actually the software you added on top that caused it.

On the other hand, I didn't have much of a problem maintaining multi-week uptimes simply due to the fact that I kept the system relatively "clean" and wasn't trying out new apps every day.