If you are on a YUPP fare then you are usually on a fare basis Y, which is pretty much top priority for stand-by. I was at the gate in chicago for a flight to LA and we had missed our earlier flight due to weather. We were therefore stand-by. There were something like 44 passengers on the standby list 2 hours before departure, and being on Y fares were were #1 and #2. That stayed the case right up until flight departure even though the standby list grew). We ended up taking the last two F seats on the flight (those held back for full fare).
Yeah, I agree, (although, there are various xUPPS that are booked primarily in lower classes, not always Y)
I think the summary is: "Book it, accept the small risk, but counter that against the huge saving you make."
Waitlisting is fun though. You do get some interesting situations:
"Are you FAA certified? We have a jump seat available".
"Are you happy to accept a seat in Economy? If so, I have one available"
(~20mins before flight is due to leave you arrive, and the list for upgrades is huge). "Yes, there is one seat available in F."
It's a combination of the fare type, your OW status and whether you are a distressed pax (ie disrupted) that affects where you are on the board... But it really shouldn't be an issue for most people.