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.
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.