It's true that SharedArrayBuffer is coming, but there's no way to share objects from what I understand. Since Doppio maps JVM objects to JS objects, it cannot take advantage of shared array buffers to emulate shared memory threads.
For interop with JavaScript, there's a usability difference between a JavaScript object and a blob of bytes, although that could be overcome with an object "mirror" that proxies operations appropriately.
Our approach was to leverage the existing GC and language features that browsers already have.