I worked without pay for 6 months. I had no indication anything was wrong. We raised a seed round were about to finally start paying ourselves the cofounders booted me. Suddenly they weren't happy with my performance, though days before they'd praised it. Worst of all one of them still hadn't quit his full-time job!
The vesting cliff protects those who stay from a founder leaving early, but it also creates the possibility of a founder getting strategically booted once the business is less risky and/or starts getting traction. To be perfectly honest after going through this, I'm not very inclined to do a founder vesting cliff again.