I know it’s not relevant yet since Ubuntu 18.04 doesn’t go “live” for another 2 months. But my experience with 17.10 (with xorg) and 18.04 have been less great. For me it works fine as expected on lower resolutions, once I get to 1680×1050 it mostly works OK, but occasionally will hang or crash, and 1920×1080 doesn’t work at all, just visual gibberish on the screen. This happens both with VP8 and H.264.
It’s nothing I worry about all too much since they aren’t officially supported and I didn’t expect them to be either (17.10 being wayland by default and 18.04 still in beta), just thought I’d add my experience in case someone stumbles across this thread and thinks it should work fine out-of-the box based on the user feedback above.