One messy thing is that it appears that assignment via .coords is broken on DataTree even at main.

Ah yes - I forgot there's a TODO deep somewhere for that 😅 I left it for later because it seemed like it might require changing the DatasetCoordinates class that ds.coords returns. (Surely that class could just be replaced by the new xr.Coordinates class now??)

Originally posted by @TomNicholas in #9063 (comment)