MOS "itself" is IMO at least as stable as AOS 3.x.
Also AOS users say that application crashes are often not able to take the system down.
This is also common with MOS/AOS 1-3.x, depends on how the app bombs whether or not it crashes the OS completely.
Anyways, my advice is that the Pegasos (using MOS) is a great hobby system, and lots of fun if you're looking for an amiga-like system "with a twist" of something unique.
Also I would say to buy any of these systems (A1/Peg) for what they can do today (or RSN), because, although hopes are high, the future isn't exactly promised.