When the window manager is a separate process with async communication between the WM and display server things can get out of sync for a frame or two which leads to visual artifacts. In Wayland the window manager works synchronously with the compositor so that it's never out of sync.
Yeah, that makes sense. It seems like instead of introducing another IPC protocol like this project does, there could be a compositor that loads different window managers as plugins. Then everything is in the same process and there is no need for async communication. Of course a crash in the window manager would take down the compositor, but this is already true for Wayland compositors that combine both.