(If this has been posted before, sorry)
I hadn't tried using the influence strategy until now. Generally it is less "exciting" to me. But tryign it out, I have found that it is altogether too easy to flip a planet. All I have to do is build culture buildings, research a little influence tech... and before long, invariably, my colony out influences the AI, and their planet becomes mine.
First, I think it ought to be more difficult to do this.
Second, there are no repercussions that I am aware of. After flipping a planet, AI civs don't become more mad at me... or don't consider it an act of war. But it seems inane that any civilization would simply let a planet join another civilization without a big stink. Now, if all you do is play the diplomacy and influence hand, even with a terribly weak military, I can almost waltz my way through to an alliance victory. And while I have nothing against such a strategy in general, I would think that there should be greater resistance along the way.