> But none of the examples you mention, to my knowledge, provide multi-key ACID transactions

If you see it that way, you are right. The guaranty is just for one key/document pair (the key can be a key vector though). There is no way to have a commit over several key/document pairs with these databases.

If FoundationDB can do that, it is a big plus. If it can do it in a cluster, your product will have a great future.