Where it falls down is mostly in documentation, which is 100% understandable, since it was crowdfunded(!), but the code is very legible and amenable to most kinds of webapps.
Where i suspect draft.js probably shines is having tighter client/data boundaries and code that natively expects to be rendered isomorphically, so calls to window/document are probably more tightly managed given that the whole view layer uses react and likely expects to be rendered on the server.
That said, ProseMirror has a 100% fantastic "give me the current ast rendered as markdown" which, in my case at least, was the thing i wanted the most and that ProseMirror supplied handily. Draft.js will have you roll your own[1], but at least the draft.js api is fairly friendly.
There's obviously more than enough room for both projects, but i think that for react based projects, draft.js will feel like a more natural fit even if it will take more effort in the short term until more functionality builds up around it.
[1]: https://github.com/facebook/draft-js/blob/06891490069d3ccfb8...