I always hesitate when someone says there's "no downside" to accepting large sums of money.
I trust Paul implicitly, and I suspect most YC Founders do, and that probably has a lot to do with the acceptance rate. I wasn't in the meeting obviously, so I can't know how I would have reacted to the specifics, but I can say knowing only what I know now I probably would have at least taken the weekend to do my homework.
Edit: phrasing.
What rational reason would an angel investor have to avoid investing in a company that has taken this investment?