Nicky B is definitely evil, but he's far from Yawgmoth's level. He's a damn power-hungry and super-wizard tyrant, but he is more of the power broker type. He ruled the empire of Madara back in his first life and was, well, not content, what with him tearing up whatever parts of the continent earned his ire, but he inherently had way more power than Yawgy did and did far less with it.
Even as he did his nefarious schemes through his agents -- and he did plenty of them, including hijacking the entire plane of Amonkhet to be his personal zombie army factory -- he was way more of a string-puller than someone who forced people to convert to him on a large scale. Tezzeret and Vraska escaped his servitude with themselves (mostly) intact, and he even honored his bargains with them.