At this point, joining as a seed-round or series A employee seems like a sucker's bet if you're expecting equity to be worth anything.
Many IPO-track companies are growing fast and attracting serious investment, so they pay engineers top dollar. They want to maximize chances of getting to IPO so they don’t benefit from cutting corners on a few engineers.
If you have The opportunity to join an actual IPO-track company then go for it. The catch is that many startups will claim to be headed for IPO, so you have to look at actual growth and TAM and make your own judgments.
Even in the optimistic case of an exit, whether it be an acquisition or IPO, the risk incurred by the employees is way too high because of the lengthening of time towards the exit event. The lucky ones can get non-recourse loans and cash out with something to show for their efforts, but the overwhelming majority can't, and end up having to shell out real money for what are essentially rolls of the dice or losing that equity altogether.
It's interesting that if a person has vested stock with nominal value of $100 and exercise price of $5, he can't use that stock in lieu of the $5! i.e., he can't walk away with $95 worth of his grant. I wonder if there are IRS regulations that bar this type of arrangement.