I feel high quality 3D is one of the final things that we need to master in the browser. Once this is done, I can imagine PWAs get more widespread adoption for gaming and AR etc.
Since the browsers black list GPUs and platforms where they have issues, as author you don't have any guarantee how the users will get to see your site.
Additional not all GL ES features are exposed to WebGL.
Depends on your definition of fine. In certain cases it would crash browsers on load, in other cases it would deliver a solid 60fps. I think 3d on the web does need a bit more polish so that it's a good experience universally.
The Unity engine has been outputting WebGL builds for years, and having played a bunch of them (and deployed a few myself), I've never once seen a problem that didn't exist in other builds of the same game
Kudos to the good people working on it!