I think, your points are largely correct. You indeed need to lock multiple buckets when you commit data which could be slow.

My point is that you can tolerate that if your application is something like gmail. Client A has his own tree saved on the server, not intersecting with the tree of client B. Client A probably is not going to write from multiple places simultaneously too often.

But if you have groups of clients writing to the same tree, I think it's better to have some writing client which handles multiple commits of others.