They're way, way overengineered for the very limited use cases they're designed for.

If you're going to ditch the browser API's and re-write the world in Javascript, why not take into consideration all of the history of GUI development and do it right? I.e., make a real layout engine, proper widget hierarchies, real encapsulation, etc.

Instead there's some sort of frankenstein in-between state where you've ditched all the benefits of native browser API's but haven't arrived anywhere that's a true paradigm shift.

TL;DR - yeah, they make some things easier to reason about, but for any sort of non-standard webapp you'll still need to write an equivalent mountain of hacks and framework adaptors.