While Bishop + Fortress is most assuredly stronger than Bishop+Chapel, it will also guarantee a no-win situation for both players in a mirror match. With an 8 card deck of 4 Bishops and 4 Fortresses, a player can churn 12Vp per turn without draining piles. In one turn, the deck can be modified to 3 Bishops and 4 Fortresses to start draining $3-$2 piles for 8Vp per turn. Problem is of course that if both players go for Bishop + Fortress, no one is going to back down from the 12Vp variation.
The only situation where I believe this deck could be beat (besides any other cards that can stuff the golden deck) is in a 4 player game where the other three start racing for the provinces quickly. That's 4 Provinces per player rather than 8 for the second player in a two player match. Even still, the amount of Vp amassed by the Bishop + Fortress player before that happens would probably be too much to overcome.
I suppose conceivably if two players in a three-four player match went for the same combo, the third (and fourth) player could empty the Bishops and Fortresses...but then what third pile could he/they empty while gaining sufficient points to overcome the former's setup period?
Simply put, Bishop + Fortress is quintessentially the best and worst non-attack combo in the game right now. I believe it's a strategy that cannot be turned down barring interference from certain attackers, but it's also one that no one can win with.