I'd say there are a lot of creators, depending on the content type. As long as you're not looking for travel vloggers or unboxing/review videos or "lifestyle" vloggers there's plenty. Pretty much all science/history/urbanism/aviation YouTubers I follow are on Nebula as well.

Issue is that the UX isn't great (e.g. there are no playlists nor queues, and downloads don't work as well as they do on YouTube).

I agree, most of my YouTube creator's are there. I'm basically missing only Tom Scott, Kurzgesagt and Technology Connections.

UX is pretty meh (although the "Nebula Enhancer" browser plugin at least fixes the missing queue), but that doesn't work in the app obviously