I have flown United with my disabled father on United from LHR to SFO and back.
My experiences were mixed. The crew were helpful on both flights, particularly the London based crew on the inbound. They were proactive with little features like offering my mum and dad a cup of tea without them asking.
Boarding on the other hand was appalling. We were not allowed to preboard due to some problem with the computer at the desk. When we were allowed to board we were sent down with my dad in his wheelchair with all the other passengers. We had intentionally picked a seat next to the toilet and it was right by door 2L. Both 1L and 2L were in use and we were made to use the airbridge to 1L as this was for premium passengers. When I asked to use 2L I was told do you want to get on this plane or not.
Of course when we went down 1L the flight attendant had to get the aisle wheelchair out to the inconvenience of my dad and the other passengers.
Do I blame Unite, well not really, its all about individuals using common sense.
Maybe United's culture is not to allow people to use common sense.
I can contrast this with my boarding and transfer experiences with AA who were absolutely brilliant.