It isn't OP, because you need to consider: 1) like was said that some rooms would be useless otherwise 2) all players have the same access to these rooms, so there is no unfair advantage 3) there is a lot of work and planning that goes into a room, you need the tunnel first and then it cannot be adjacent to another room. 3) point scoring rooms are only in the second year. Think of all the value (and point potential) that you got from your production rooms since year one. You NEED to make a lot of points from the year two rooms to make them even worthwhile. 4) you can get the rooms, but nothing is guaranteeing that you will keep them. (Stupid adventurers.....)
Look at the scoring card. First it says each unconquered room gives 2 points, and then near the end you score point-scoring rooms.
Rooms are usually powerful, but also expensive. You often have to pay a gold for one, and besides the restriction on where you can build it you have to replace a tunnel (where you can mine gold). There might also be the event "Extra Tax" in which case the room becomes even more expensive.