Clearly, you should be skeptical of anyone offering a "surefire deal" on anything. Skepticism is healthy and good.

But suppose the founders of Airbnb came to you when they were raising angel money. I'm sure they were convinced at that time (maybe objectively irrationally) that investing in their company was a "surefire deal." They were passed on by Fred Wilson (the equivalent of your "rich person with hundreds of staff"), among countless others I'm sure.

By this point, their returns are certainly far more than 12-15%.