We're using Slate to for a mid-sized react application where content editing is the core. I don't want to link/plug a product video showing you how it works but imagine dropbox paper, medium editor, CKeditor 5, ...
The implementation includes mentions, pretty complex gallery/inline image functionality, galleries with size/output controls, embeds (iframely) and other inline elements.
First we had TinyMCE, Redactor, DraftJS and then switched to Slate.
I would not say it's smooth sailing from here, we're mostly struggling with pasting from word, tables, mobile support and overall performance with the number of plugins, but working on it.