I have only flown on AA and DL, not UA, but they were miles apart. Found DL similar to if not better to European airlines in terms of service (just the food that was worse). Once the capt'n introduced themselves personally to everyone in J.
I have also flown AC and have good memories from staff interactions.
AA staff were unhelpful and up to downright rude, which has pretty much never happened to me in any other airline, ever.