NDA == Fail

I have had the same experience, and I would definitely say there is a strong correlation but it is not a case of NDA -> Fail, but more like a Fail -> NDA.

Specifically, the type of person who thinks they ONLY have an idea, and that execution does not matter, and that there are no other problems to be solved besides coming up with the idea---that person is highly motivated to NDA.

Meanwhile, the person who thinks there are a lot of problems to be solved of which one very important part is the product... that person is less motivatd to NDA because they don't think you can copy their idea, ship it a week or two ahead of them, and beat them.