Probably badly, it's true. There is, of course, some considerable dilution of the much larger volume because there's many legacy airlines in the US - AA, DL, UA - before we even begin to consider the LCCs, so the effect may not be quite so pronounced when the numbers are spread around.

On the wider point, I tend to agree that UA in Y is at least as good as BA in Y on LH, and possibly better given the relatively small cost of buying up to Y+ for the extra leg-room. If you want cramped without an option, try AC on the aging 767s!

I've flown UA in Y many times through corporate travel policies and even through personal choice. I've avoided UA J because, up until very recently, the cheaper fares didn't earn in A3's FFP and prices generally weren't any better than BA. I tend to think flight crew are often what you make them, especially down the back - I've never failed to be polite and sympathetic, and in return I've always found them to be helpful and friendly, whatever the airline. If I were a passenger who was only buying on price with a bit of a regard for service level, stepping on a UA plane wouldn't remotely bother me.