This is a long list of complexities, but I'm not sure how they relate to udev/systemd. Yes, setting up aggregate interfaces, VPNs and fancy drive configurations is complex and can fail due to various delays. That's not a problem with udev reliability if an underlying system fails. We had the same issues before systemd/udev was a thing.
> cryptsetup hangs for swap devices because those get a key from /dev/random
You can choose urandom (third column of crypttab). Systemd doesn't enforce anything here.
You wanted to know how many udev events and therefore dynamic behaviour a server could have at boot. I told you my experience.
And of course the underlying system may fail, but actually it is more common to have udev, lvm and systemd fail. Just google cryptsetup, udev, boot, lvm, raid and problem in arbitrary combinations. Most of what you will find are bugs in udev, lvm or systemd.
urandom is a workaround for stupid behaviour from systemd. Of course /dev/random isn't necessary, but that is beside the point, systemd again broke something by being boneheaded.
> cryptsetup hangs for swap devices because those get a key from /dev/random
You can choose urandom (third column of crypttab). Systemd doesn't enforce anything here.