The irony is that most of the newer ones in Linux are written that way, just nobody writes a FUSE binding to them. For example, both Btrfs and XFS contain fully functional pure-userspace implementations of the filesystem inside their userspace code (libbtrfs and libxfs both contain the complete filesystem code), but nobody has written a FUSE binding to either.
Btrfs' implementation is even used to do most of the functionality in the tools (like send/receive) rather than using the kernel to do it (in order to minimize context switches that reduce I/O performance).
FUSE is neat for one-off access to obscure file systems, and FUSE is usable for things like sshfs for those who use that, but it is my impression from having used FUSE myself that FUSE does come with a noticeable performance penalty. Is my perception wrong on this? Because if it is not then I’d be hesitant to use a system where the main storage was relying on FUSE.
FUSE is also great for slower media. Copying files over USB sticks without being restrained by FAT-32 limitations is great. There have been times that I've used NTFS-3g for this because its FUSE implementation meant I could read it on any OS.
The benchmarks on their website[1] compare it running on a 4 x 3.20GHz with hardware AES-N instructions, against an in-kernel implementation of a different cryptosystem running on a 2.7GHz cpu that does NOT have AES instructions. I also can't see anything that speaks to speed of the storage (or if this is controlled for), so I'm admittedly quite skeptical.
The numbers they do publish aren't particularly impressive to me either, so we may also have wildly different ideas about what "quite high performance" even means.
Btrfs' implementation is even used to do most of the functionality in the tools (like send/receive) rather than using the kernel to do it (in order to minimize context switches that reduce I/O performance).