What all can a dyno do?
What a dyno cannot do?
A dyno cannot fix mechanical issues with the vehicle nor electrical issues of any kind. It may be used in discovering them, but neither the dyno or laptop will fix or tune out mechanical or electrical issues.
A dyno won't print out the same power and torque number when compared to other dynos nearby. The power numbers will vary from dyno to dyno due to countless reasons like an inertia dyno vs. a loaded dyno. Other scenarios can stem from faulty weather stations giving false correction factors, wrong dyno calibration parameters, dyno's physical and electrical condition, and much more.
The Dyno also requires a dyno operator that has a great understanding of how a dyno works and must be able to operate the dyno properly. No Dyno Manufacture "Patented" the perfect dyno, nor the Laws of Physics. A dyno is a tool in the Tuner's toolbox. Plain and simple!