"Unlimited vacation" is horrible. It's not good for employees, but it's awesome for employers. The main problem is that despite the word "unlimited", it isn't unlimited. That's because there's still the concept of "what's a reasonable amount of vacation to take", which is a limit.
The only reason anyone would change a reasonable amount of vacation from an objective measure (x hours earned per pay period, for example) to a subjective one is so that those in control of the subjective definition can benefit.
You, as an employee, certainly have no power whatsoever to define or enforce what reasonable means. It comes down to the opinion of your employer—your boss, or his boss, or her boss. Or whether you take more than the other employees in your group (even if they're taking none and you're taking less than you used to). And if your boss likes you today, great, you can get more vacation. But if your boss doesn't like you tomorrow, well, now you've abused the system and are in trouble if you take that much vacation. Who's to argue one way or another? Think about it—your employer has no contractual obligation to let you have any vacation at all! Of course, you can take it, and then suffer the consequences.
When you leave the company and haven't taken all the vacation you wanted, you don't get it now—try telling your company you're leaving in 2 months and wish your final 6 weeks to be all vacation time—but could you take it as a lump sum, so you get paid now and don't have to come in again after 2 weeks from now? See how that works for you.
If someone else plans badly or there's simply more work than a single person can do, so that through no fault of your own you don't have time to take vacation and meet certain unreasonable or unrealistic goals, this policy makes it your fault for taking vacation that you see as reasonable in its timing or length, but even just one person in power doesn't agree. In retrospect. When you couldn't have known at the time. Because how much vacation is reasonable is subjective and can be reassessed after the fact, at any time, for any reason, at the whim of the employer.
Frankly, and objectively speaking (with hard data available out there that will back this up), companies that switch to "unlimited vacation" always see the total amount of employee vacation taken go down. Anyone trying to sell you on the idea that it's better for employees is just selling you a bill of goods.
A better description than unlimited vacation is indeterminate vacation. You have no idea how much vacation you'll get, and whether it will hurt you to take any.
How would you like to go to work for "unlimited pay" where you choose your own pay rate week to week, but your employer decides if you are abusing the system—and if so, you're fired? Would you feel safe? Or would you ratchet back your pay just a bit to achieve that safety? So similarly, it would be an "indeterminate pay" system, which to me sounds like a horrible, horrible way to work. And if the company's fortunes turn, or they're bought by another company, who's to say that doing the same job to the same quality level next year as this year will warrant the same pay in the eyes of your employer?