1. It's effectively unmaintained (which I found out when I started mailing around asking if there was interest in me sending patches that fix the rest of the issues listed).
2. It requires privileges to use, making it useless when compared to other alternatives that can help solve some of the other issues (pidfds or just the good old /proc/$pid fstatat(2) trick).
3. It doesn't work in containers at all.
4. It has several pretty serious bugs which could even be argued to be security bugs. But since it has effectively zero users now, I'd be surprised if anyone would be interested in such bugs.
I wanted to fix these issues quite desperately, because it would allow for init systems that don't suffer from the cgroup or ptrace downsides. Unfortunately, it uses netlink and so any changes are mind-bogglingly complicated (especially if you want to tie it to PID namespaces because then you're really SoL since netlink is fundamentally tied to network namespaces).
