Systemd incompatible with...

Oh I'm sure that all the internal busses are PCIe and if there is an external ExpressCard slot, that's hot-plug PCIe. When it comes to power management I imagine that some devices drop out and show up on the bus in a hot-plug fashion when they are powered off/on.

> moved udevd [...] without leaving behind a compatibility symlink, breaking booting ...

That's pretty screwed up in any event. I find the logic and evidence compelling for collapsing / into /usr but that is no excuse for not putting in appropriate compatibility measures, measures which should be totally straightforward and transparent.

> The udev maintainers' motto appears to be "backward compatibility is for other people"

That's an unfortunate motto of much (though thankfully not all!) of userspace, it's one area where having source available has lead people to believe that rebuilding and recompiling the world is acceptable so API/ABI compatibility don't matter.