I have used LH upgrades a couple of times, but had UA agents refuse them on two occasions, because they had never seen LH upgrade certificates before/didn't believe they were valid. In addition, one UA agent told me "I don't have time for Lufthansa upgrades, next customer!" Another UA gate agent said "Keep your original seat, an upgrade on those [x2 LH vouchers] isn't going to happen!" So I went to the RCC with them and my upgrade did happen!

Needless to mention: These problems disapear, by using UA vouchers on UA and LH vouchers on LH. I don't blame UA or LH for looking after their own elite customers first. Judging by the reactions of airline staff there don't appear to be too many UGS/HON or 1K/SEN's around.