I think the problem is complexity. A compositing window manager should not be terribly complex and it should be reasonably possible to get all the bugs out over time. One problem is feature creep. Features that were not part of wayland due to security concerns are being implemented anyway - warp pointer, color picker, etc.
I found the missing features in Wayland problematic -- especially not being able to share windows during video conferencing which eventually forced me back to X (on Fedora 27).
> Features that were not part of wayland due to security concerns are being implemented anyway
Yes, because without those features it was secure and useless.
Those features are being implemented in secure ways. We can control who gets to use those features now.
How do Windows and MacOSX go about this? Do they get it correct? If yes, how? If no, what are the workarounds they use?
Windows manages just fine, symptoms of crashed GPU driver are black screen for couple of seconds until you get your desktop back.