With a powerful guarantee, response goes up, but the number of demands for a refund does not significantly increase. General claims sound ho-hum and are less believable. It is much more effective to be as specific as possible. Your message will get lost without clear explanation of benefits. Never assume that your prospect will automatically understand the benefits to be gained from your product.
Benefits are the only reason anyone buys anything.
A benefit-oriented ad will out pull a feature oriented ad every time. There are many elements to advertising. In addition to the headline, there's the layout, medium, placement within a medium, mailing list, timing, etc. No one, not even an expert can predict the best possible combination. Sure, a knowledgeable marketing professional can make intelligent recommendations.