Nothing really. Except that the risk is all yours. You have to pay, dearly, whether it works or not. And when the money runs out you simply disappear off your customers radar.
The most effective marketing in the traditional marketing sense is quality PR, followed by targeted below the line campaigns. In the online world there are two types of  marketing.

Effective, and less effective. The less effective may bring you more leads but at a price that makes the whole process unprofitable. Online advertising campaigns such as pay per click or adwords are great while the money flows. Or they should be. But when the budget runs out, where are you left?

What good PR is to above the line advertising, so too is organic optimisation to online advertising.

The budget money runs out, but your site is organically positioned to continue to be in front of your potential customers’ eyes. Organic optimisation is not SEO. It is a structured marketing effort that combines the right platform, targeted copy,indexed keywords per article, a structured site and a social media support campaign. Organic optimisation is a bit like the roots of a tree. Essential !