I’m sure the process isn’t entirely fair, since picking winners at such an early stage is incredibly hard and they have so little time for each candidate, but I’m also sure that they _try_ to be as fair as possible.
And I write this as someone that has been rejected about half a dozen times...
https://www.youtube.com/watch?v=rfTgzA6iKZc https://news.ycombinator.com/item?id=21288988
Note: He was one of the partners assigned to us and invested in our company with his VC firm.
I would presume the farther along your business is and the more stuck you are on that particular model the more the evaluation leans toward the business growth metrics.
When we did YC we were accepted with an idea, a landing page with some email signups, and almost a customer. They asked hard hitting questions in the interview. We did have industry connections, some unique insights, and previous startup experience.
After being "inside" (as a company founder at least) and seeing everything they write about on the "outside", I take what YC says at face value. They earned trust.