This was my understanding and experience.
Blocking estrogen during PCT can lead to big problems.
Guys have been using Clomid/Nolva combos for years with no problems.

I think the big problem is the misuse of AI's all together. A lot of guys overdo it and drive their estrogen into the ground. Proper use of an AI during a cycle and even in PCT can help manage aromatization issues that some people experience, it's not for everyone, but works extremely well for guys who use it properly.
I've seen a lot of people dismiss AI's all together but I think the problem is, just like with everything else in this game, there's a real lack of understanding on how to use AI's properly and that using a dose Joe Blow posted on some board for his cycle as your own dose isn't the best way to go about it. Plus there are different AI's all with different levels of response, i.e. Letro being very potent compared to say Aromasin all of which have varying levels of effectiveness at many different dose ranges.
I'm beginning to stop recommending use of AI's to newer people, with the exception of those that are very gyno prone, because I think it's really something they should run once they have a few cycles under their belt and know how they react without an AI first.