There was a separate comment about Draft, so I answered that one there and will just comment on ProseMirror here.
ProseMirror's main is insight is the need to maintain a document model in parallel with the actual DOM. While most modern editors also do this, ProseMirror also allows their document model to be extended and customized, which only Quill and possibly Draft[1] is also capable of. I think this is the right idea and foundation to build on top of (Quill also made this choice).
I don't think it's fair to judge implementation or specifics beyond its ideas just yet since it is still in the development stages and things can change and improve, as stated on its README:
"NOTE: This project is in BETA stage. It isn't thoroughly tested, and the API might still change across 0.x releases. You are welcome to use it, but don't expect it to be very stable yet."
[1] "Possibly" because Draft uses inline styles and only two levels of content (block and inline) so it's far more limiting.