There are exceptions. One is the topic of this article: people who are able to move up a rung faster than they would at an established company. Leaving a grunt programming job to be a technical VP is a valid reason to pick a startup. Another is to learn how to run your own startup - of course, you'd have to be going to a position where you'll actually get that experience. Heads-down hacking for twelve hours a day doesn't teach you to run a company.
It's the rank-and-file employees who usually get the short end of the stick in a startup. There are the obvious drawbacks - low salary/no benefits/long hours - and the possibility the company will fold up. Unfortunately, there's no real upside for the average employee. The average startup is not going to hire on enough staff that early hires are going to have much room for advancement. Assuming the company reaches an exit - almost always selling the company - the employees won't get a big payday, but they usually suffer in the acquisition. I've been through several purchases, and it's usually not a happy process unless you're waiting for a large check. Even if the acquirer wants the startup's staff, the founders and a lot of the staff will probably be gone in a couple of years while whatever was attractive about the startup's culture is systematically disassembled.
Of course, many startup employees probably don't see it this way. Startups thrive on young grads looking to preserve the sense of purpose, freedom, and esprit-de-corps of college life. After all, if the last year has shown anything, it's that most people don't make decisions like strictly rational, self-interested economists.