They are like option grants that last about 7-10 years, that exercise on IPO/acquisition and don't let you exercise before then.
So with these companies, all of the tax complexity goes away, and you don't have the 'if I leave after 2 years, all of my stock options go away after 90 days' dilemmas. You don't have the rich man option of pre-exercising to potentially get some long term capitol gains tax savings although. If the trump administration gets rid of the AMT stock option trap, then you will also not have that advantage with ISO-type stock options.
Since IPO is the realistic condition for these companies, you also will have the 6 month SEC lockup period. The company might add their own additional lockup period afterwards too.
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The attraction about joining these companies a few years before probable IPO is you have not insignificant potential to increase your income above the average tech worker income level. Facebook is the success example, twitter, fitbit, etc are the not as successful example. You can also get similar effects by joining apple before the iPhone was released or during the early iPhone years.
It's definitely a risk, but isn't as risky as joining a startup where you can't see an IPO in it's future any time soon.