Unlimited PTO doesn't mean unlimited PTO

I see a lot of articles on the subject of unlimited PTO, but most seem optimistic and don't seem to address my concerns.

Full disclaimer: I've never worked for a company with this policy. However, I've been around the block and have good instincts when it comes to some things.

The upsides have been covered ad nauseum. Here are the concerns I have.

1) You hear the talking point that you can take all the vacation you like "as long as the work is done". My concern is that the expected amount of work is poorly defined and/or you may be caught in a cycle of perpetually missed deadlines. I worked for MegaCorp for 15 years where completely unrealistic deadlines were the norm. You'd go from one late project to another where you are behind before even starting. How can you ever claim that the work is done in that environment?

2) Revolving managers. This is another issue I had at MegaCorp. Getting assigned a new manager (very common there) would often mean a complete upheaval of your work/life balance. The new manager may take a closer look at the PTO you are taking, leading to increased anxiety during these transitions. With fixed vacation time, managers can come and go, and they don't have much of a say about that.

3) Competition with coworkers. Your coworkers should primarily be your teammates, but they can also be competitors. You will compete with them for offices, promotions, compensation, fringe benefits, and even keeping your jobs when the pink slips are handed out around Christmas time. Who gets to keep their job...Hank who took four weeks of vacation, or Walter who hasn't taken a vacation in two years?