FUSE doesn't compose well, though.

On the Hurd you can, for example, transparently access a node of an XML file inside a gzipped tarball on an FTP server; with FUSE you'd have to design the userspace file system adapter to support these extensions, whereas with the Hurd it's a consequence of using nodes of the virtual file system as flexible rendezvous points for stacked translators.

With FUSE users are still second to admins when it comes to file systems. On the Hurd all file operations can be interposed -- it enables fine-grained virtualisation by design.

(On Linux, on the other hand, user namespaces are a post-hoc hack.)