If someone hears "mmap is faster than system calls" and then mmaps and munmaps 4KB chunks at a time in a loop, not realizing that mmap and munmap are actually system calls and that the benefit is not about calling those functions as much as possible, there is no saving them.

That's not the fault of a 'dangerous' claim, that's the fault of people who go heads first into something without taking 20 minutes to understand what they are doing or 20 minutes to profile after.