IMG has posted a news item that answers a few questions about the Mac version of Myth III, as said by MacSoft's Nate Birkholz in the IMG forum. His post:

Just so people are up to date, the A-Level bug list for the Mac version is as follows:

1) Random crashes under OS X when using an nVidia card. Traced to GeForce drivers for OS X.
2) Unexplained performance drop under OS X around build 134 (PC gold version).
3) LAN networking broken under OS X.
4) GameSpy broken under OS X.

Mumbo Jumbo has been working hard on getting these issues resolved. While they are working on these, they are fixing the Mac-specific B-level bugs and folding in fixes and tweaks reported from the PC version.

Birkholz also stopped by the Myth Village forum with some fantastic news:

Myth III under OS 9 is now rock solid. We can't crash it at all, and we've played for hours of multiplay (cooperative and competitive) without crashing, dropping, or otherwise experiencing nastiness.

MacSoft seems intent on releasing a solid application that won't be plagued with nasty bugs like the PC version. If this is all true, Mac users can look forward to a much smoother release than the prior one.