Personally I don’t see a scenario for myself, but I can imagine that there are some where this might be useful.
But isn’t there a extremely high risk of data loss an inconsistency when adding an extra layer on top of OS file system handling?
Freqfs seems like a shim you'd add to an existing project for a quick optimization. Whereas mmap et al. are "better" the same way any specific, built-to-purpose code will be "better" than just bolting a framework on. Sometimes it's the right call to do the extra work; sometimes it's 100% more effort (both development and maintenance) for an extra 10% gain.
