The chance of prizing 4 UBs in a buzzwole deck is .003%
The chance of beast ball even having the potential to grab ANYTHING in a buzzwole deck is 35%. Even then, having an item card that can search buzzwole is not even close to being overpowered.
Unless they release an Ultra Energy card that lets you run like 15 UB's, then most decks aren't going to run more than 7 or 8 ub's, and in those decks, Beast Ball will be almost entirely useless in 40-45% of games, and that's at least 1 UB. If you want at least 2 UB's in your prizes to use more than one beast ball in a game, then there's only a 17% chance if you run 8 UB's. In the games where it's USABLE, it's not amazing.