Biggest factors, I would guess, are the particular agent, how busy they are, and how full the flight is.

Some agents are just more accommodating than others, although I don't know you can say that ORD and EWR agents are less accommodating than those at IAH and DEN. Seems pretty arbitrary.

If it's a very full flight, don't expect much, if anything, to happen on the ground. It may be an onboard effort, in which case remember to offer something better to the person you're hoping to switch with. Don't pull the "hey guy in E+ aisle, can you switch to E- middle so I can sit with my friend/family member?" schtick. What would work better is offering like for like (aisle to aisle) or better (middle to aisle); moving the person forward a few rows (especially E+); offering to buy them a drink, snack box, wifi, whatever; and/or offering cash to switch, especially if it's a worse seat (aisle to middle)

If there's weather and delays, other IRROPS, misconnects, a standby list a mile long, etc., don't expect much, if any, help on the ground. They're too busy doing operationally critical tasks to be bothered by managing nice gestures which ultimately make things more complicated.

That said, I've seen it happen, I've also seen it not happen. Don't think you can draw any conclusions about UA in this regard.