I will be probably switching from quill to prosemirror because quill ignores numerous requests for shadow dom compat. It also annoyed me that its hard to get rendered delta results from it - we had some workarounds for it, maybe they provide an API for that nowdays.
Currently it seems to not support customising and/or creating your own blots (like here — http://quilljs.com/guides/how-to-customize-quill/#customizin...)
Also, while we're at it, syncing operations (and example of which is shown here — https://www.slatejs.org/#/syncing-operations) was a hard concept to wrap my head around until I found this — https://operational-transformation.github.io/visualization.h...