There are no guaranteed results with any genealogical research. No one can guarantee to find something unless they already know it is there. No one can guarantee to solve a genealogical problem unless they already know the answer to it.
All a researcher can guarantee is that they will research the records to the best of their ability and accurately report their findings. Of course when one hires a researcher, they want to hire someone who has experience in searching the records; is familiar with how the records were created, organized, and maintained; and is skilled in their interpretation and analysis.