Pro's of the extra slots:
1. Clearly visible and noticeable to the player.
2. Immersion as they get to see the actual building in question depicted with its own graphic.
3. UI. By this I mean, the the tool tip for buildings with too many effects will cut off the screen partially. If you've played DeI, you know what I'm talking about as some of the main settlement lines they use have this small problem. This would potentially come into play.
Negatives:
1. Lose sack/loot. While this can be compensated for, it's still cutting features that could probably be developed further. Included in this - natural disasters (which are more annoying than anything) would need to be cut from the game, as well.
2. Stability questions. I haven't had any issues, and I don't know what would cause any.
I'd love to have more buildings, but this is the direction I'm leaning. Daruwind, I know you disagree with me on the number of 'unique' regions, but I think that has more to do with communication breakdowns.
Ultimately, unique doesn't equal easier for the player or AI. The building chain impact on the game is just a matter of the effects we set. So, regional unique areas could have the effects of the extra buildings we had planned on while being standard in every other way. Or they could be customized further.