I guess there are quite a few negative experiences. Esp those that were rejected by Y Combinator. I think Y Combinator is over-rated- atleast now (I am attracting downvotes-Go ahead) Their startups havent had really big exits, so far.Once they do,things might change.
I don't think you can count 'rejected by YC' as a 'negative experience'. It's pretty much the definition of 'non-experience'. It's like saying that the department store experience was bad because they wouldn't sell you a lawnmower at half price when you asked them to.
The fact that I didn't build the company I was pitching is prima facie evidence that I agreed with their assessment. (and in retrospect, I wasn't the right person to do that company and you could hear it in my voice)