How are concurrent updates to the same file handled? Either only one client can open in write at any one time, or you need fencing tokens.
For concurrent updates, the standard practice for remote file systems is to use file locking to coordinate concurrent writes. Otherwise, NFS doesn't have any guarantees about WRITE operation ordering. If you're talking about concurrent writes which occur from NFS and S3 simultaneously, this leads to undefined behavior. We think that this is okay if we do a good job at detecting and alerting the user if this occurs because we don't think that there are applications currently written to do this kind of simultaneous data editing (because Regatta didn't exist yet).
Consistency at the individual file can be guaranteed this way, but I don't think this works across multiple files (as you need a global total order of operations). In any case, this is a pragmatic solution, and I like the tradeoffs. Comparing against NFS rather than Spanner seems the right way to look at it.