Regarding spinal mounts in general, I feel like it would be nice if you could actually have multiples of them, and the main tradeoff is purely the fact that you cannot turret them (so your tracking speed is limited to the speed of the ship). I think that would be a big enough tradeoff (they can be bigger but there is no way to turret them when they are built that way), the ship would need to be pretty fast to make them effective (whereas you could have a slow/stationary thing if you stuck to turreted weapons).
I also want to say I strongly favor the idea of shield shock damage wearing down the emitters, I think it makes sense that you would be beating up on the things if you are blasting away at them with huge nukes or lasers of equivalent power level. It also would be pretty entertaining if you had damage control crews frantically trying to keep the shields up as the generators blow out.