It's unlimited until 2020[0] and then I'm guessing you'll have to pay for it afterwards, and after a few years of taking videos without caring about space, you're probably going to rack up a massive bill or have to spend a ton of time cherry picking what you want to save.
Their plan is pretty smart tho. Most people will be like "screw it, I'll just buy another phone to avoid spending that just to keep my videos".
[0]: https://i.imgur.com/M5LYvrH.png (read the fine print)