I think what you mean is preferred stock. And (a) the distinction only matters if the startup does badly, in which case it doesn't matter much, and (b) the founders don't have preferred stock either, so an employee is in the same boat as them. (It is vanishingly rare for individual employees to get their stock/options taken back. Zynga is the only recent case I know of.)
It is certainly not true that joining a YC company means joining one post VC funding. Even if it did, it doesn't matter, because (as investors know) the startup you pick affects returns way more than the stage at which you pick it. That's why VCs are willing to invest in B and C rounds at high valuations when they could invest in other (less promising) companies' A rounds at much lower valuations.
As you say, I did mean preferred stock. You caught me before my edit :)
I still stand by the assertion that investors are looking for something different than the typical employee. It seems to be a large risk for a large reward; I don't want to presume to speak for you, but usually investors are both looking for a large exit that an employee will not really benefit from, and can be involved with many companies at once to minimize their risk.
I definitely agree for investors that the startup you pick is more important than the stage its in when you pick it. But that's still involving a significant chunk of the stock even in later rounds. Ultimately, unless the employee is getting a large chunk of the company (in which case he should think as an investor and would be wise to go for YC companies) the decision process should be different.
It is certainly not true that joining a YC company means joining one post VC funding. Even if it did, it doesn't matter, because (as investors know) the startup you pick affects returns way more than the stage at which you pick it. That's why VCs are willing to invest in B and C rounds at high valuations when they could invest in other (less promising) companies' A rounds at much lower valuations.