The thing is, with X there were a lot of separate parts - the display server, the window manager, and the compositor were separate packages and if either of the latter crashed and you were set up for it you could keep going. With Wayland, the display server is the window manager and compositor - and is directly responsible for handling all audio, video, input devices, color management, clipboard, and probably other things I'm forgetting - so there's only one thing to crash and it's game over. This also is a pain in terms of features, because you can't tack on, say, a clipboard manager, it has to be directly supported, hence things like GNOME, KDE, and basically everything else (via wlroots) having different, incompatible ways to do things like screen shots.