Crashes of the now-merged display server / window manager / whatever. Why does everybody keep talking about "applications" like they're the problem people have with Wayland? Our problem is the Wayland server itself.
Both window manager and shell were historically extremely likely to crash, whereas the display server was very resilient (and the session script would restart the WM and shell as needed). I'm not sure how separate the shell is nowadays, but more than just the traditional duties of a WM have been embugged into the display server.
A single crash of what? I have used a Wayland only system for a long time and application-level crashes certainly don't bring down other applications much less the whole desktop.