Anecdotal after using Wayland for the last few months:

  * The brightness control for my screen doesn't work with it.

Never been an issue on my Samsung laptop. I strongly suspect this depends on your hardware, but I'm not going to spend a week to prove it.

  * The mouse sometimes lags

Never happened outside of catastrophic load scenarios.

  * Graphical corruption in Chromium when on webgl sites.

Are you using Nvidia? I've had no major graphics glitches since using AMD.

  * Blank screen after resume from hibernate.

Never happened.

The last two aren't applicable, since if anything has been OOM killed recently I didn't notice, and I don't have a Chromecast.