I started rewriting gcsfuse using https://github.com/hanwen/go-fuse instead of https://github.com/jacobsa/fuse and found it rock-solid. FUSE has come a long way in the last few years, including things like passthrough.

Honestly, I'd give FUSE a second chance, you'd be surprised at how useful it can be -- after all, it's literally running in userland so you don't need to do anything funky with privileges. However, if I starting afresh on a similar project I'd probably be looking at using 9p2000.L instead.