It's all anecdotal of course, but I've found flights from BOS have a decent chance of priority tags working. I've seen BOS ramp have a separate cart planeside full of only priority-tagged bags, set aside. They load that after all the other bags. I've had consistently good luck BOS-DEN, including my bag being #1 on the carousel last week BOS-DEN UA465.

More typically, I've found my bags coming out in the first 10-30 bags. Not perfect but ok.

When they do the process right, it works. Like anything UA these days, there is a lot of inconsistency from station to station.