I don't hate United, they're actually my airline of choice and I've held their 1K status since 2018. Based out of SFO and a frequent flyer its really the best choice as they hold close to 50% of the flights out of SFO. Maintaining 1K is totally worth it as anytime I call in I get a representative within 30 seconds, and in case of irrops I'm basically guaranteed a seat on the next flight.
My to biggest complaints are cleanliness and the service on board. I'd say neither of these are BAD, but there are other, better airlines.
Cleanliness: If I had to estimate, I can note cleanliness issues on a third of my flights on United. The worst is looking in the seat pockets or looking at the air vents above the windows (don't look there on any airline its usually disgustingly full of dust).
Service: As for service on board, the flight attendants are a little more rough, less attentive than other airlines on the same route. A common route for me is SFO-NRT and ANA flight attendants are checking in every 5 minutes during dinner service, where as United FA often disappear. The ANA food is of quality I would pay for not even on a plane. On United, I often get offered "chicken or fish", no paper menu, no table cloth, even in business. I think some of this is United being slow to bring back a higher quality of service post-pandemic, whereas ANA never dropped their quality of service.
I used to fly regularly from OZ to the US and often had to take United as they used to often be the cheapest by quite a margin. They had the oldest and most uncomfortable cabins on those routes. The cabin crew is absolutely atrocious, everytime they come through the cabin shouting "elbows and knees" (why is that so common on US Airlines? Never heard it anywhere else) and still manage to bump into everything. I have had coffee spilled on me at least 4 times and not once did I get an apology (and no I didn't have my elbow out). If you ask for something you often get rudely told off.. The food is absolutely the worst I ever ate on a plane and it goes on.