I pulled one of classes from my project that does not depend on anything external and fed it to their demo site. It threw up bunch of errors complaining that RuntimeException is undefined. It seems it doesn't understand built-ins unless you prefix them with \, even though they are imported properly with use statement. This is pretty core issue to lack support for. Calling it "beta" is actually giving it a lock of slack, I'd say it's closer to being proof of concept.