It's amazing to me what companies will stick in these contracts, and how deep down the rabbit hole they'll stick it. It's their stock to do it with, of course, but it's just annoying that seemingly employee-centric companies will do such seemingly abusive things. For example, I've seen instances of sale restrictions being
four contracts deep (e.g., "shall be governed by (x) agreement", and that agreement says "shall be bound by terms in (y)", and so on) in an agreement that employees were only ever officially given a draft version of but apparently still held as effective. It should have never been signed of course, but the obtuse nature and comforting language these things are couched in can be confusing.
At the same time, it's also unfortunate that people don't do research to look at other instruments for liquidity (like pre-paid forward transactions) that the restraining company has zero control over. In Uber's case though, it seems like they're actually paying on the up-and-up. Many companies intentionally deflate the fair market value of shares far, far below the actual valuation (like 1/25th the value paid by investors in the last round, for example), so offering to sell at what the investors paid in at is pretty decent.
Private equity is confusing and usually doesn't work in your favor. My general advice is to always appreciate it, but never depend on it as part of your compensation in any way (and Good Lord, don't bank your retirement on it!).