As far as my experience goes, all of the official content (base game, expansions, and smaller DLC) is fully playable on the current version of the engine 😁.

OpenMW is still marked as 0.x primarily because of the editor - while the game runtime is in fully playable state, OpenMW-CS is still lacking some features compared to the official Construction Set, and the parity of the tooling is required for them to mark the project as 1.0.

Of note, while many mods work fine with OpenMW, not all of them do - some break on bad scripts (OpenMW is notably more strict on compilation errors), and anything requiring the use of the executable extenders (MWSE/MGE) is not compatible at all (adding support for the functionality they provide is considered post-1.0).

It's completely playable, and with the Tamriel Rebuilt mod, you get actually double the original!

It's definitely playable. I even got it to work on my android phone with little trouble, though I wouldn't recommend it without a mouse & keyboard setup.