Here's another one: When should/shouldn't you exercise options when leaving a startup?

Obviously, if you're leaving Uber you'd be wise to buy your vested equity if you can.

Obviously, if you're leaving a company that is crumbling, you probably shouldn't.

But what about an earlier, Series A/B startup that's promising but still shows a lot of risk? Or what about a later stage startup that is doing well but not on a guaranteed path to a huge exit? On what information would you decide whether or not to exercise?