Oohh… I thought you were the first to signal the collaborative editing issues with OnlyOffice, but now I learn that you are otherwise happy with OO.
I agree that we shouldn’t jump too fast 
And what issues did you have with CollaboraOffice, in what instance? Of course the editors are very different, CO implementing parts of LibreOffice and using it in the backend, using ODF as core format. While OnlyOffice may be generally more mature, and using Microsoft formats as core formats. There is also the client side processing in the case of OO, in theory preferrable as Chris mentions.
But for collaborative editing, which is the key usecase of these editors (otherwise you can just edit on your local office application), concurrent editing should work at least with small groups. And the current setup doesn’t allow for that. So please suggest what would be your preferred way forward.