dropboxforum thread here: https://www.dropboxforum.com/t5/Installation-and-desktop-app...
I mean, the Linux manpage itself says it's stable and can be used, with the inode number, to uniquely identify a file. http://man7.org/linux/man-pages/man2/statfs.2.html
I'm not surprised, though. Fancy copy-on-write filesystems like btrfs have some other subtle gotchas. If you allocate (like, really, not as a hole) a big file on btrfs and mmap it for write, you might see a SIGBUS upon writing because btrfs needed to recompress a block and the new compressed block didn't fit where the old one did.
I've gained a new appreciation for the predictability and simplicity of ext4.
Once saw a system that used inode numbers as "unique identifiers for files on local disk" and it turns out that on Linux / ext filesystems, they're not unique (they get reused if you delete a file and then create a new one). That team just decided to not support Linux at all.