no lah mine isn't a UF8. UF7, but no problems on it. i've got a friend who bought a UF8, which i remember playing for him with some hard-hitting playing in the showroom. ran through as many of the 88 keys as i could, don't recall a single one being deficient. especially any of the keys in the vicinity of the middle C.
might have been a bad batch of units from the factory, if it's a common problem among several users.