I think a lot of it boils down to luck of the draw. If your flight is delayed by several hours, you're not going to remember or care whether or not the entire airline is otherwise having a good day operationally. General patterns can emerge, and I would say that UA is running a good operation these days, but people are always running the risk of running into IRROPS on any airline, with some notable outliers (WN over Christmas versus everyone else.)
FWIW, I've flown SFO-DEN r/t three times in the last two months, and the worst thing I ran into was an hour delay last Sunday night because of deicing. But I also think I have good luck that I almost always fly mainline, fly a lot of hub-hub routes, and rarely need to connect.
Maybe CommuteAir is still having trouble?
Commuteair has been good for me. The only issue I had was that I was not aware the the E145 only has extra oxygen masks in 5 rows, so as we were boarding with our 11 month old, we were handed new boarding passes for the last row of the plane. I think that could have been handled better, because we were left to explain to the people who were supposed to be in row 24 that their seats had moved. Apparently rows 3,7,11,and 21 also have an extra mask, so I don't know why we couldn't be switched from row 4 to row 7? Why did it have to be from a "premium" seat to the last row, next to the engines and bathroom?
But while otherwise the E145 is one of my absolute favorite planes, I was disappointed to learn that my hated CR2 exceeds it in this way, and has an extra oxygen mask in every row but the exit. Luckily with Premier Status, I didn't pay for the premium seats, it was a short flight that got in early, and the flight attendant gave us extra stroopwaffles for the problem.