"UAC never requires a password"
Unless you configure it to do so, which is trivial. If you can learn how to configure sudo, you can learn how to configure UAC.
"plus it has the same problems of losing network connections, kicking-off VNC access, etc as logging-off and logging-back on."
UAC is a completely different process from login, and there's no reason why it should lose network connections. I've seen a couple of reports of UAC causing network connections to drop, but no signs of any actual evidence UAC was the problem. As for VNC, if UAC on the VNC server is the issue, configure it to not use the secure desktop for the UAC prompt (which makes perfect sense, given how VNC and UAC work).
Granted, the OYS ("Over Your Shoulder" - what Microsoft called the heuristic automatic invocation of the UAC mechanism, back in '05 when they introduced it as Limited User Access) "feature" is a pain in the ass, particularly since many of the heuristics are idiotic. (It can be triggered by running a no-manifest executable with the letters "inst" in its name, for example.) But overall UAC is not nearly as bad as you claim.
And, of course, you always have the option of not running as a UAC-controlled administrative user on Windows, and using runas when you need elevation. Just like the other grownups.
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