Notably, I think you still can't get hardware acceleration under Wayland for nvidia cards, making it unusable. Yeah that's nvidia's "fault" but to the end user it makes no difference if they upgrade their distro, the distro switches from Xorg to Wayland, and suddenly their computer sucks.
Nvidia wrote patches for GNOME and KDE to support their crappy custom EGLStreams thing alongside the usual GBM. I think GNOME and KDE have accepted them.
However, xfwm (the window manager) will also need to be ported to Wayland — i.e. become a Wayland compositor — which will probably be a more complicated undertaking, as "xfwm-wayland will need to fulfil the role of both X and the window manager. (Yes, using libraries like wlroots or libmir, will make this not completely impossible, but it will still be non-trivial.) Plausibly, XFCE might join forces and use the same Wayland compositor as, say, LXQt or Mate.
https://wiki.xfce.org/releng/4.14/roadmap#roadmapplanned_fea...