This may be being said elsewhere but I think the bigger problem is that you get taxed when buying options that are not liquid. I own a significant amount of a startup company and am fully vested but I can't exercise because of the tax bill.
You can. You just can't do long term capital gains.
This article seems to be of the same opinion.
Either way, if you can't do capital gains and you're still at the company no use wasting the money on buying in early.