This stance is absurd. Either you trust him to be loyal, or you don't. That's what it comes down to when you're dealing with high value assets like goddamn Flesh Forges. In light of every interaction between Qyburn and Viserys, where Viserys has not only approved of all of Qyburn's experiments including the man turning himself into something beyond the Mind Flayers, but also giving him imperial funding to do it? Why on earth would Qyburn start sneaking around behind Viserys' back now to do experiments that according to you would jeopardize everything he's achieved when instead he could just pursue other far more interesting projects with imperial support?
Leaving aside loyalty, he's not a blithering idiot. That's a bad bet and he knows it.
So if you don't trust him, why are we trusting him to be in charge of a Flesh Forge? You can't half ass this. Giving him enough rope to hang himself is not an argument when he could do so much more damage with a Forge at his beck and call than he ever could with a few acolytes following him around.
I do trust him, generally speaking.
This is a bit hard to explain for me, but I hope it comes over as coherent:
Qyburn is not a perfectly rational actor. He values his science and his self-improvement higher than his life, higher than his sanity.
This is a proven fact, he pursued his studies already back when he knew it could cost him everything, back in the Citadel.
Later on he drastically altered his own mind, essentially destroyed parts of himself for the sake of self-improvement.
Also he risked his life, mind and soul with his transformation.
So, I do trust him to be diligent in his research and I do trust him that he would never try to turn our fleshforges against us.
If only because there is not much to win for him, he does not care for temporal power and he would abhor the demands of rulership even if he somehow conquered a Forge and the area around.
But here's the thing, in his drive he does very likely come upon ideas that Viserys or maybe Vee as the most directly involved person would not approve. Something, just as one example, that would require the creation of sapient test-subjects that exist only to suffer and die in a manner relevant to his research, which is one of the few things Vee would forbid.
And he keeps away from things that are forbidden, because he is smart enough to know that it is not worth it, that the odds of getting away with anything are too far against him. Not out of loyalty, not out of morals, merely out of pragmatism.
Giving him loyal minions that could do some work outside the highly controlled area of our Fleshforges would give him opportunity to pursue an experiment or two without approval. If he is clever about it (and he would be), then he could maybe do something in reach of the Uncertainty Wards and it's unlikely it would be discovered.
So he would be back in the same situation he was in the Citadel (of course in a far better situation in general, but still), he could chase his further perfection and forbidden knowledge at just a relativly low risk of being found out.
Don't you think there's a chance he'd take the risk? Don't you think we should spare him the temptation?