Google AdWords isn't perfect, but it's more than what you mentioned it is. Knowing that you'reasking for the disadvantages, here's several that I can think of.
It's easy setting up an AdWords campaign. But this being an extensive tool, you'd need to set it up carefully to be cost-effective. That being said, to set up everything just right for your campaign, you might need the help of some experts (if you can't on your own).
Each click to your website is a charged to you even if that visitor buys or not from your website. And if you fail to pay for AdWords can cause your presence from Google to be immediately removed.