I flew a LOT pre-covid, and still more than average post-covid (2-4 times a year), and primarily via United (thanks to racked-up miles from the Continental days) out of Houston/NYC/London, and I have never had any issues with delays or cancellations - In 2009, a Continental flight did have me circling IAH for 2 hours while they troubleshot a landing gear malfunction before landing again and switching planes, though, but that is the only issue I remember, and I was flying 2-5 flights/week at that point between HOU/NYC/LAX/DEN.
The handful of times I've flown Southwest have been slightly less than perfect, but some of that was user error not understanding how Southwest worked compared to normal carriers.
I don't want to discount yours, or the thousands upon thousands of reports about United or Southwest, but in my experience, it has been pretty solid on both counts.