On which one does it work? I haven't run across a Linux filesystem where it doesn't work, including mounts over NFS. But I suppose on some non-POSIX platforms you might have more issues, or with some esoteric mount options to screw with mtime or something.

The reason it's disabled by default is first of all that it's relatively new, but secondly that if your FS stops behaving as expected (or you move your repo to one that doesn't behave) it'll degrade very badly, i.e. "git status" might now completely miss files that have been modified in your working tree.

But if you're just running a system where you know you can trust the FS you can use the untracked cache and get a lot of "git status" speed-up, which'll matter more the larger your checkout size is in terms of checked out directories & files.