Asiana Airline itself is really really great, but its FFP - the Asiana Club - is simply one of the worst. (But then again, believe it or not, Korean Air's Skypass is MUCH worse.)
It seems there are lots of benefits being an UA elite member, but Asiana offers nothing special to diamond or platinum members. (Never heard of any free upgrades or vouchers)
Maybe it's because Asiana is already very nice to its customers regardless of their membership level. I'm not even a *A silver member, but they gave me once a free C upgrade for Beijing-Seoul track.
Other than those standard *G benefits there seems to be none further from Asiana.

Maybe long flights simply belong to usual life for US people, but they are (still) something special for Koreans. It has to be inter-continental for them to be meaningful. Both Beijing and Tokyo are too close to Seoul (about 600 and 750 miles each)

I have a Citibank credit card which gives me 2 Asiana miles/KRW1000 (USD1 = KRW950) which is very nice. And fortunately, I know a "dirty" trick to legally purchase up to 12500 Asiana miles per month for about USD105 resulting in getting the ticket for less than half the normal price.
Plus, I have an LG platinum credit card which gives me the highest grade Priority Pass as a bonus. The annual fee is about USD52.
In fact, I got that platinum card solely to get the Priority Pass. :-)

So I'll certainly keep my Asiana Club membership and each time I fly I'll make a decision where those miles are to be registered : Asiana, Spanair, or maybe BMI.

cya