So, one of my players made an unbelieveable 6-success-roll yesterday night. The action card he used just showed up to three hammers, so he was a little frustrated that there was nothing he could have done whit the additinal three successes.

Do I miss something or is it really that way, that additional successes just buff out?

Yeah that is a decent roll but not a great roll. You really want some variety. In running the demo Friday night our troll slayer would up with 3-4 successes and lots of eagles. He was able to use the 3 hammer line, and his 2 eagle line and get a hit for like +6 damage and 2 crits.

You can only use one success result, the biggest being usually three successes, any more are discounted. However boons can trigger more than one result. If you had rolled six boons and no banes, you could have triggered any or all of the boon results on the action card, triggered a weapon critical, and/ or removed a fatigue or stress, as long as the total number of boons used came to six or less, and no boon result on the card was used more than once.

One of the cool things about WFRP3 and the dice pool. The "best" rolls are not rolls that maximize success symbols. They are rolls that have a lot of successes as well as a lot of boons.