This is nothing new, they have been doing this for 2 years or so IIRC. Anyway, for at least 1!
Actually the process seems to work rather well. You have a lot less confusion in the boarding area as there are many fewer announcements to make... especially on 763 and larger aircraft! The gate agents seem to be very good at letting each group get into the jetway (at least) before calling the next group. As the seating area is very clearly marked on the boarding pass and the process is well explained in announcements there is no mad rush to the door as the first few rows are called. The agents are human and I have never seen them force a family or pair of travelers to split: the highest boarding area will apply to a whole family (the computers may even mark all the cards together if booked on the same itinerary, i don't know though...).
And I think most of all, the antsy passengers are put in their place: at the back of the aircraft and the back of the line! Think about it... while all airlines give priority to their elites, normally inviting them to board whenever, UA
pretty much keeps them seperate (in actual practice the elites are seating area 1). The premier crowd knows how to get aboard, get stowed, and get seated... and they get to do so without the hassle of seldom-flyer who gets lost at row 12 looking for row 21.
And a final point, it might make sense to board groups of rows to the rear, but you are very likely to have someone take the aisle seat, only to need to get up and step into the row to let the window-guy in. Now the ailse is jammed. In reality if I stop at row 10 to put my stuff in the bin, even on a singe aisle jet someone can squeeze past me to get to row 25 (especially since his butt most likely won't brush some aisle-guy's face!).
Bottom line, I've flown some thirty segments with UA
this year, and only one twice do I recall departing late. Those times were due to waiting for connecting pax to arrive from tight connections or waiting for all the baggage to be loaded. One other flight was slow, but on time, but I blame that one on 3/4 of the pax having no clue what an airplane was. (Some vacation group, i'm sure, headed to HNL
on a 772... I swear the ones who didn't step into the starboard aisle after being directed by the FA
and stop to ponder exactly how far back their row was (which they had forgotten anyway so had to look up) for a full minute... all had decided that the exit row seat by the 2R
door was the best in the house and they would take it (until shooed away by a FA
or their companion who had pondered just how far back their assigned seat was!))