Copying from my email reply:

Chrome OS uses FUSE to support certain filesystems like NTFS (via ntfs-3g) and exFAT (via exfat-fuse), and certain archive types like RAR (via avfs).  FAT and ext23/4 are still supported via in-kernel module.   Another use of FUSE is the virtual-file-provider, which isn't quite related to the discussion here.

I'm not sure if sdcardfs is relevant in the context of cros-disks.  We deliberately chose a user-space FUSE solution (instead of a kernel implementation) for supporting certain filesystems / archives on "removable media" like USB sticks and SD cards such that we can sandbox the FUSE helper like other user-space processes. Performance hasn't been a reported issue on removable drives. But we always want to minimize the risk of a rogue USB stick leading to a kernel exploit.

We are aware of fuse performance issue (chromium:731497)
sdcardfs is not working well with container and arc++ (https://buganizer.corp.google.com/issues/62847046#comment17)

We are looking at using esdfs, which is more container friendly.
However, esdfs will not be used in P (happy with sdcardfs), nor Q(where a newer implementation of sdcardfs is slated - https://android-review.googlesource.com/c/kernel/common/+/525636)

I need to reupload esdfs from https://android-review.googlesource.com/c/kernel/common/+/522727/10.

Re #3:  gwendal: that seems to be the emulated FS for ARC++, not particularly related to removable SD cards / USB sticks. 

Re #0: dianders: should we update the bug description to ARC++ specific?  I'm still particularly clear how sdcardfs would be relevant to our support for NTFS/exFAT on removable drives.

OK, I think the base problem is that I was just confused by the email suggesting that we move from fuse to sdcardfs.  I didn't know anything about sdcardfs, but that made me think that it was some better way for doing the same types of things that fuse does.

...but actually digging into what sdcardfs seems to be about, it's obvious that sdcardfs would definitely not be useful for cros-disks's mounting of NTFS / exFAT and it's not some "better version" of fuse--it's just trying to replace what Android used fuse for...

---

I'm going to just say that we WontFix this bug and our ARC++ team can track things themselves.  I originally filed the bug mostly to find the right owners and make sure we were looking at this, if it was useful.

Thanks everyone!