Personally, i haven't found them all that overpowered; at least when i used them. In a custom battle I used a full rocket fleet with a single 3rd rate admiral, and the AI used the preset fleet, and nothing i could do would win the battle for me. My problem was that the rocket ships wouldnt start firing until relatively close, so they would only get 1 volley off before the enemy line was upon them anyway. After that, they just blew me to pieces.
I take it this doesn't happen on Multiplayer?