write/fsync can be faster in a large dataset because writes let the filesystem know an explicit list of dirty pages, so fsync only needs to deal with them.

mmap/msync gives no hints about which pages are dirty (unless the app tracks them itself and msyncs them individually, which would completely defeat any reduced syscall advantage of using a writable mmap in the first place) so the entire map must be scanned for dirty pages.

In practice, the expected performance advantages of using a writable mmap just aren't there, and coupled with the ease of silent corruption, it's best to never use that approach.