Less code runs as root than with X, since the server is far more lean.
There should be zero tearing or artifacts in Wayland, and the latency should be lower as well. Both due to passive compositing.
Screensavers are a pointless waste of energy. I'm not sure why you mention them.
In Wayland, WM/DE use IPC to communicate with each other; not X.
In theory you can run multiple XWayland servers to separate from each other. If security is a concern, Qubes might also be an option. And you're still more secure and better performance with Wayland plus some XWayland than with X.