How hard is it to maintain and keep secure an entire modern, complex, evolving file system which the OS itself does not even use (so nothing like simple, frozen

How hard is it to maintain and keep secure an entire modern, complex, evolving file system which the OS itself does not even use (so nothing like simple, frozen, and interchange-standard FAT) in an operating system kernel? Very hard. Instead there are 3rd party implementations. Those are independently maintained, serving directly the (comparatively small) target audience.