I suppose the mento-intensifier could have different levels of success depending on the particular mind it was trying to affect.
And it might have unwanted side effects if used injudiciously... I mean, perhaps he could have taken full control of Maya Hansen at the risk of reducing her mental acumen.
If you're creating completely obedient guards and warriors, that's no problem, but messing with the mind of a genius you want to work for you at the top of their game.
Or maybe the Knaufs just thought that The Mandarin should be an awesome manipulator, and choose to give the rings a power-level that would supplement that without drowning it out or rendering it irrelevent? You can't really present him as a brilliant manipulator if the rings are so cosmically powerful that he doesn't actually need to manipulate anyone.
It's not a coincidence that Fraction's Mandarin has a more powerful mind-ring but Mandarin himself seems dumber. It's a natural push-pull that happens one the result of the other.