Some of the complaints about sd_notify is that it is just an implementation on top of $NOTIFY_SOCKET.... which is completely undocumented. So you have a decentl

Some of the complaints about sd_notify is that it is just an implementation on top of $NOTIFY_SOCKET.... which is completely undocumented. So you have a decently documented and small implementation by Lennart that relies on something that might as well be hoodoo magic unless you've gone through the undocumented code. There's also problems with nscd as pointed out in the comments, but the FSF is insane enough in general that I don't think that's necessarily a problem with systemd and more of an issue with working around the labyrinth of bullshit that is the FSF and their policies.