Bribe judges is very nice, It is perhaps the best ability in the whole game - and if I was going to change it, the only way I would would to perhaps make it cost 3 yen instead of 2 yen. The sash however can only use bribe judges a limited number of times in a match, a point well worth remembering. A sash with yen coming out of their ears can still only bribe so many times before they have to play the game honest.
As for dirty trick, it is not supposed to be a game winning power - it is supposed to be a game delaying power. Personally, I think that dirty trick is more easily used in a strategy match than in a normal match. For one thing it enables you to go last which is very useful in a strategy match. Second, it allows you to pick what you think your opponents best move is and get rid of it. Timing is everything however with dirty trick. If you use it while you have a game winning move in your hand, and yes we do all know that some moves are better than others, then you deserve to lose anyway. The point of the powers is, after all, to put a little more control of the game in the hand of the players so they can make intelligent decisions that help them win. It often works best in the 3rd round a hand, but if you are in the first round of the first hand and you are a yakuza fighting an honor fighter who just used fist of fury with a signature style in each of the two moves then it makes since to sacrifice a single good move to prevent a possible four point hit.
I have always enjoyed having dirty tricks as a villain and especially in strategy matches. used right it will make your opponent tear out his hair in frustration. And Vanor is exactly right. If you could choose yourself which move to discard then it would be too powerful.