I am using Slate Js and it's fantastic, though there are some caveats. Mobile support is limited, slate is still not in a stable release so there may be breaking changes, and for now you're limited to using react. But when it works, it's amazing! I really like the freedom it gives you in defining and changing your own document schema and how everything is rendered.
I think the biggest fault is a lack of virtual rendering. Slate renders the entire document so if you're doc is too big, slate starts to break fast. Virtual rendering (like in the Ace Editor) solves the problem.
