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).
