Although I have unfortunately had several experiences with UA similar to the OP, I do have a few flights that really stick out in my mind as excellent. In particular, a FRA-IAD flight in 2007 with a Paris based crew (Is the Paris crew base still open?). Between a cabin full of businessmen and a surprisingly all male crew, my wife was the only woman in the J cabin. The crew paid her ridiculous amounts of attention (which she loved), and then thanked me regularly for letting them do so. Overall, really excellent, professional crew. I wish they were all as motivated and service-oriented.
Paris crew base was closed quite a while ago. IIRC, this happened around the time when SFO-CDG non-stop flight was slashed. Some of CDG based crews transferred to FRA, and you will still see them on ex-FRA flights. I had a flight from FRA to IAD in C last year, and there were more French crews than German crews in C cabin.
Regarding SFO-ICN-SFO, FAs are not cool in general. However, a lot of them are senior crews. (I heard senior SFO crews favor this flight due to short turn around time of just one day) We all know that FA pay scale at UA is not cool at all, and they must be tired with just one day layover at Seoul. So what do you expect? No wonder why they are not cheerful to serve customers at all.