But, you have to pay income tax on the value of the shares at the time when they vest, i.e. become non-restricted. You have zero control over that vesting schedule, and thus the tax bill, and you likely wouldn't have a liquid market for the shares before an IPO.

Any gain post-vest can indeed be long-term cap gains, if you hold the shares > 1 year.

You would only pay capital gains on the gains from that stock. At RSU vest you would owe taxes on the market value of those RSUs.