Potency and Tenacity - how do the mechanics actually work?

There's a reason people do a quick tenacity-potency calculation before sending in their Nest vs Padme (even using Farmboy Luke lead to increase the tenacity if neccessary).
There's a reason people use a potency cross on their JKA when they want to beat KRU and DR teams.
It all comes down to the team comps and their modding.