Quill is already over 10 years old! I've deployed it in multiple applications with a lot of success.
Customizing has been harder than anyone would like. I became an event sourcing aficionado, initially to support parchment’s delta format. It’s a solid idea, but also required (unless you do backend diffs, which I tried too).
My vanity test: I can’t believe how hard it still is to make a TODO checklist with indented levels. I try like every 18 months, surveying the plugin ecosystem.
It never works well, so far. Editor.js isn’t a one-stop rtf editor, but that was the first thing I noticed “just working.”
That said, I’m upgrading an old prototype to quill v2 today.