Admittedly, I only flew on United once (coinciding with how often I've visited the US so far), but in my eyes the complete boarding process was a horrible inefficient mess. They seem to regularly overbook flights to 150 % or so and then first ask any passengers whether they want to go on a different flight maybe since there are too many passengers. Then seemingly everyone brings baggage that
should have been checked (like, gigantic suitcases and bags) as carry-on baggage, so they ask whether anyone of those would rather have their bag put on the flight as checked baggage. All that takes plenty of time already since usually no one seems to be willing to volunteer. And then comes boarding in five different classes, most of which are distributed evenly across the plane, so the different boarding classes seem to be more about who likes to get their head bashed with baggage while they're already sitting, getting up to let people through to the window seat, and, perhaps the most useful, overhead storage bin space.
Admittedly, boarding in groups is common everywhere, although I haven't seen that many different classes until then. But I haven't ever seen overbooking to that extent or the fight for every bit of cabin baggage space to that degree.