I can't think of any disadvantage that would outweigh the huge price savings. Some UA promos require roundtrips for a flight to qualify, like the recent one where you fly x number of trips and get a one-time discount of between 5-40% depending upon how many qualifying flights. Gee, he should spend an extra $2k or whatever so he might save $50 on a domestic flight.

Nor does it seem like this would be an end-to-end or out-of-sequence issue.

I say it's a good find! ^