Isn't the main reason for knfsd being in-kernel that it depends on inode number to file lookups? Does samba have the same problem?
The kernel nfs server is IME more robust than userspace options like ganesha, but that seems to mostly be a function of it being widely used - a kernel samba server doesn't have that advantage?
The post indicates this is not to replace samba, but to improve the file sharing bits of it.