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).
ES5 took me months to understand well. And today, after years (and now on ES6/TypeScript) I cannot always properly describe some features of ES5 without seriously reviewing them.
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).