How hard is it, actually, to build a Y Combinator company?

Two kinds of hard

Practical execution, regulation, market structure, geography, operations, carries more of the score than raw technical difficulty, because it accounts for more of what actually varies between companies. Most Y Combinator companies aren’t deep-tech; regulatory and market complexity is where the real spread in difficulty shows up.

The two dimensions are almost entirely independent of each other, building hard technology and operating in a hard market are, empirically, two different problems, not two names for the same one.