Nowadays there is a big problem - too many marketing companies promising to promote your product to the first positions. But often you spend a lot of money on marketing, and the result is zero. Why? How can you check the actions of the hired marketing company? Basically, you can't. You can only hope for their honesty.
Actually that's not only a problem for hiring marketing companies, but anything related with trusting someone, you need to trust them. That's why don't be naive for trusting too good to be true or human words, you need to think with your brain.
Example like signature campaign in this forum, it's one of marketing strategy to promote a project. If the manager trying to convince the project team like: if you hire me, your project will earn $10M in a month; if you hire me, your project will get 1B new participants etc which is completely bullshit.