No disagreement as to the qurikiness of this feature (which for the record is pretty much completely unique to UA). IME, the problem is usually related to the reservation itself though for some reason, not to the general feature on the website. The littlest thing causes it to choke, i.e. a pending schedule change (even for 1 minute) on the reservation, a thru flight that has a biz cabin on one leg & a first cabin on the second (i.e. flights from MEX that continue on), a paper ticket, ual.com's mood ring for the day, etc.

I will also say that recently, even phone sponsoring of upgrades has gone through without issue though - even if you don't sign the bluey (I had a couple chances to put this to the test where I was travelling w/ the sponsoree). YMMV of course.