Another downside to unlimited PTO is if the company decides to lay you off. If PTO is defined, then in California, they have to pay it to you. If it is unlimited, then there is nothing they need to pay you.
This is important. With unlimited PTO, you no longer accrue PTO hours per pay period. Those unused PTO hours are legally required to be paid out at your usual rate when you leave, regardless of reason. I'm actually surprised that this isn't mentioned in the article at all.
Which is why, when in the context of unlimited PTO, it is sometimes suspicious to take a long (2 to 3 weeks) vacation. It is a hint that you are taking advantage of the PTO before leaving.