I've been taking Mountain Citadels, because I like mountain fortresses (heh), because I want to maximize the defensive abilities of my fortresses, and because I'm curious to see one in action. As usual though, I'm playing against the AI with my main goal to have fun and try things, rather than to minimax.
I expect the expert players are probably right that the costs could be tweaked to better reflect their value, although I'm sure Illwinter have considered the values, and may have some thoughts that some people haven't considered.
I like the idea of being able to pay more start points (and, less than the sum of each) to be able to build more than one type of fort during the game.
Another suggestion: it'd make some sense if Mountain Citadels were restricted to mountain provinces, or at least, if they weren't available in all province terrain types. Seems like you'd need a mountain (I know, what a realism freak, right?).
PvK