When I was an employee of a subsidiary of Infospace, my RSUs were always worthless (honestly, I don't remember if any vested while I was there), at Yahoo, we could generally trade, although one shouldn't trade immediately after earnings, but I don't remember if this was enforced at the affiliated brokerage. At Facebook, I think it was typically a three week window every quarter.

Of course, if you quit, the windows are no longer in force, although if you have material non-public information, you're still not allowed to trade. Maybe there'a a share price where you'd rather quit and sell than hold on until the window opens.