For me personally, the first dealbreaker I keep hitting is the fact that input-method-adjacent keyboard logic is quite broken (necessary if you ever want to use a language other than English, even temporarily).

There are also, of course, the innumerable minor bugs that will be fixed from one release to the next (but also replaced by other bugs). The number of these decay exponentially and it will probably be a decade or so before Wayland sessions becomes as stable as X11 in this regard.

There are quite a few cases where applications auto-detect that Wayland is running and as a result run in degraded mode, intentionally or unintentionally. I remember some bug with menus not working properly ...

And scaling, often touted as one of Wayland's features, often works better under X11!