Being the first employee (or an early employee) of a startup seems to be the worst possible choice.
* none of the stability of a larger company * a salary which is likely less than market * 10 to 100 times less equity than a founder (So, in an exit, you'll make 10 to 100 times less money! Not all startups have billion+ dollar exits.) * a huge amount of responsibility - might not be as much as a founder, but still comparable * less influence and decision making ability than a founder * less insight into the finances of the business than a founder (without that insight, how will you know if that equity is worth anything?)
The only benefit I see of being an early employee is if the founder is a seasoned veteran who has already done this who you can learn from and be mentored by. But the vast majority of ycombinator founders seem to be new to the startup game.