The contact tracing is annoying, but I do have to say I love United's app for checking in online, it's by far the easiest out of the big 3. I carry a Mexican passport and Visa, and unlike Delta and AA, I don't have to scan my passport every time I check in, since it's already saved within the system, and it not only allows me to add my visa information, but also tells me where to find the number (you'd be surprised how many check in agents in smaller stations that have never seen a border crossing card before elsewhere have had trouble finding this).

Half of the time with AA and DL I end up getting the "check in at the airport" error message too...