RSU's are only restricted until vest. Once they vest, they are unrestricted stock. Uber cannot control what you do with that stock (or at least, i'm not familiar with any company that has done so, and not sure that it's legal to do so)
I think the sibling post answered the question -- RSU's don't vest until the company goes public. So, instead of getting illiquid comp, you just get none until Uber is public.
Typically this is only for short periods for higher-up people but it can affect "normal" engineers too (say you do a tech due diligence for an acquisition etc....)