And, in rare cases board members can be held responsible for the actions of the board. In that sense you have a lot more to lose as a board member than as a shareholder.

So if you think that the board might be held liable by shareholders for something in the future it is usually a prudent step to create as much distance between you and the board as a shareholder as you can so you don't end up on both sides of a lawsuit.

Is that true? Many people are on multiple corporate boards, and some boards meet only a few times a year. I've only been on charity boards, but it wasn't a particularly demanding commitment.

I guess the reputation cost is a good point, but Kalanick is always going to be associated with Uber, whether he's officially on the board or not.

Reputation is usually something that affects you regardless when you are on a board. Plenty of board members invest little time or commitment, though.