Hours are long, you're on-call 24/7.

Your co-founder might want out and take almost all the equity. Even so you still must keep helping with funding.

Even if everything goes well with your co-founder, you put a lot of money into it but your shares gradually and inevitably unvest, till 18 years later you've got 0% equity.

Even so you might be expected to shell out a couple of rounds of funding ("we're getting traction and in 4 years we'll be getting revenue").

Then, although you aren't liable for your "startup"'s actions anymore, your reputation is forever linked to it.

What's the upside?