The issue I know of is, you cannot really use waitpid(-1, ...) in the same process as waitpid(<pid>, ...), as it would create a race condition on which call consumes the pid first. You can mitigate it in some programs by doing a lookup after the waitpid(-1, ...), but still it's not really reliable for libraries to spawn child processes and use that function. See for example the notes here: https://developer.gnome.org/glib/stable/glib-The-Main-Event-...

If you need to multiplex a wait for multiple processes, it's safer to use epoll with several pidfds. But you still eventually may need to call waitid(-1, ...) to reap any forked processes, and there really is no good way around that.