This means that ISOs can probably be exercised tax free, but NSOs require you to pay taxes on the spread from strike to the fair market valuation.
If your strike price is low enough you may be able to save and afford to exercise the ISOs, the NSOs will probably be too expensive with the tax burden.
There's also a 10yr expiration on options anyway so it's possible that you could lose them even if you're waiting for an IPO while holding unexercised NSOs.