There are other issues with the syscall(2) interface too, like having to split up 64 bit arguments manually on 32 bit architectures.
And the documentation in man(2) is very schizophrenic, as it is sometimes about the syscall and sometimes about the glibc interface or quirks. Ideally there should be entries in man 2 and man 3 for some more calls (or all of them?).
Sometimes glibc makes very odd decisions, like caching the output of getpid() so it returns the incorrect value if you call clone(2) yourself.
Sometimes however there can be useful bug fixes, and lots of compatibility wrappers which are useful. But again documentation in man 3 would be useful...