It looks like the FICLONE ioctl was chosen over the copy_file_range syscall[1] for use in coreutils due to concerns about copy_file_range not preserving holes.[2] I agree, it would be interesting to know how these compare, both in filesystem implementation coverage and performance.

1. https://lwn.net/Articles/659523/

2. https://debbugs.gnu.org/cgi/bugreport.cgi?bug=24399