The cleverly worded guarantee?
I can guarantee you the moon will rise in the west instead of the east tomorrow, and sure as can be, if I state it with the absolute certainty, someone will buy into it because I "guaranteed" it.
The question you first have to ask yourself is, "Is the promise realistic?" The answer is often an obvious no. A marketing company that promises you clients is just giving you sales fluff because there is no way that they can guarantee that anyone is going to want what you have to offer on your terms and at your price. They might be able to offer you leads, but they cannot guarantee you any clients. That's your job.
The second question is, "What are they guaranteeing?" Cleverly written guarantees can promise you nothing, or they can mislead you to believe they are saying what you want to hear. An old Clorox commercial said, "Nothing beats Clorox Bleach". Note that they did not say "Clorox Bleach beats all others". That's because Clorox and most other brands or even store brands of bleach are the same concentration of active chemical. They are essentially the same.
Read your guarantee carefully, and ask questions. And most of all, if they quickly try to get off the subject of their guarantee, it's probably doubtful.
Be sure that any guarantee actually minimizes your risk. What happens if they don't perform? Then what? If nothing is stated, then it just sales talk. Do you get something free? Do you get your money back? Are you allowed out of your contract? Will you be reimbursed for income that is lost?
Don't count on the last one. As much as you'd like to see such a guarantee, these are pretty unrealistic. A guarantee is just that, and rarely does it qualify as an insurance policy against loss of business. If you want such insurance, you will most definitely pay the premium price for it.