I also found that quite a few of the libraries in the Opal ecosystem (like the opal-browser library that wraps a bunch of important native web features) are fragile and under-documented.
That said, Opal worked far better than I actually expected. I built a simple demo with it a few months ago and was pleasantly surprised by many aspects of the experience. There's a third-party React wrapper called React.rb that is quite impressive.
I wrote about my Opal demo app here: https://www.rethinkdb.com/blog/ruby-opal/
And here are the slides from a talk I gave about full-stack Ruby development: https://speakerdeck.com/segphault/realtime-web-apps-with-ret...
It's only 59.0 KB, see http://cdn.opalrb.org/opal/current/opal.min.js