I questioned this last week.
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/57620/36-tenacity-70-potency#latestIn my experience I use Daka, with 70% potency. Yes, Daka only has a 60% chance of applying stun, and when she does, you better believe I pay attention to when she gets resisted.
According to the calculations in this thread and by what @CG_Kozispoon wrote, you would need 86% tenacity in order for Daka's potency to be fully nullified. Yet multiple times against toons with 30-35% tenacity my Daka gets resisted at least 50% of the time. Yes, that lowers potency to 40%, but POTENCY being 40% should not mean it gets resisted that much, otherwise the lower potency someone has they would NEVER get a debuff off. (Characters start at 0 potency anyway.)