Not many people are going to totally ignore their opportunity costs to join as a "founder-ish" engineer at a startup.
I have taken a role like this before, but when I did so, I was making a lateral move from a compensation standpoint. This is without factoring in any expected value of equity (because there was none).
Before Stripe - I had to take a loan out, and make the switch to Authorize.net so I could get my company off PayPal and its 60 day reserve - otherwise it would have folded. They were perpetually holding tens of thousands of dollars at all times.
I have taken a role like this before, but when I did so, I was making a lateral move from a compensation standpoint. This is without factoring in any expected value of equity (because there was none).