The problem with startup RSUs is that you are taxed when the RSUs vest. If there is no liquidity (which is the case for most startups) then you're paying taxes on RSUs which you can't sell and may never be worth anything.
This is not always the case. The grant may be structured such that you don't actually have the shares in your possession even after vesting, you just have a claim on them that will be honored at IPO or change of control. This though means you can't dump them on SharePost/SecondMarket etc. even after they vest.