It is not posix anything. It provides a compatibility layer that makes open, close, read, and write work but other than that does not provide the type of features that would allow you to deliver mail on it with qmail or whatever. It is incredibly misleading to advertise it that way.

As you say there is no free lunch with distributed filesystems. Application programmers have to program their way around the fact that something like posix atomic writes with multiple writers is never going to work, and that the only way to get reasonable efficiency out of the thing is to defer work until the file is closed.