I have mixed feelings about United Airlines. Last year I flew with them from Calgary (Canada) to Chicago, where I transferred to SAS for a flight to Copenhagen and then on to Madrid.
My first impression of United was that they allow far too much carry-on luggage in the cabin. People were bringing suitcases - two each in some cases, plus a purse or laptop or both - and stuffing them into the overhead bins. The extra luggage lead to slow boarding and a generally crowded plane. All that extra stuff has to be stashed somewhere and it doesn't all fit in the overhead bins.
The cabin crew were not happy. At one point the purser reminded the passengers that they were under the direction of the airplane captain, who could ask them to leave. (Although this would be impractical once we were airborne). The passengers were ignoring the seat belt sign while we were preparing for take-off.
So... do bad passengers just bring out the worst in staff? Maybe. I don't know what it was about that United flight but the one coming home was more orderly. Still, there was an excess of cabin baggage. Why?
At Heathrow Airport earlier this month, I went through security to board an Air Canada flight. Everyone there is limited to ONE PIECE OF CARRY ON CABIN BAGGAGE ONLY. This is strictly enforced. It is done for security reasons. One side benefit is that the plane cabin feels much less crowded when there's less "stuff" aboard.