Getting an Agency to run FBA Business with me being completely hands-off

I am a guy that has many things to focus on but I find FBA to be one of the best ways to make money WITH TIME.

I am entirely new to this and was wondering i start a private label by hiring an amazon FBA agency for me who handle A-Z process for me while my hands are completely off the business. Do you know of any success stories by this approach? The positive side of these FBA Agencies (even though expensive to afford) I find is -

  1. They are experts in amazon selling.
  2. Can be able to hit decided targets faster than someone who is entirely new to this business model (Me).

I wonder what are the downsides of working with or letting a third party manage and run your amazon FBA for you.