> After the 409a valuation of the startup gets too high, this option becomes much harder for employees
(Disclaimer: I am not a lawyer, not your lawyer, this is not legal/tax advice, etc.)
As I understand it, it's not just the valuation, but also the tax implications. If the options (as granted) are worth more than a certain threshold (~$100K), you can't early-exercise them without losing ISO tax treatment on the value in excess of the $100K threshold. This threshold is on an annual basis, so not exercising them early means that you quadruple[0] the amount that will be eligible AMT (which is preferable to ordinary income tax).
[0] Assuming the standard vesting schedule, in which 1/4 of the shares are made available each year