Tolkien kiedyś wytłumaczył dlaczego dewot-Gandalf byłby dużo gorszy niż Sauron gdyby upadł i wziął dla siebie pierścień władzy:
Because unlike Sauron, Gandalf really understood the minds of the Free Peoples.
One detail in Tolkien is that all beings, even the most powerful, had defects and limitations, be they of power, perception, wisdom, or physical strength.
Manwë could not understand evil, this was a flaw in his relationship with his brother Melkor. And Melkor did not understand Pity, which was also a flaw in his judgment and war strategies.
Sauron and Gandalf of course are not free from limitations, we know that Sauron was not able to think in peaceful terms or where domination was not the goal. He never thought anyone would destroy the One Ring instead of using it. This was a misjudgment (as Manwë and Melkor had) and the advantage of Gandalf.
Gandalf was inferior to other beings in Middle Earth, he feared Sauron, he was inferior in authority to Saruman, and he knew that there were other powers in the world greater than him. But where there was a weakness there was a greater understanding of things. He was the only Istar who didn't attach himself to a place or study of things. He knew Elves, Men, Dwarves, and Hobbits better, dealing with them more than the others did.
As a disciple of Manwë, Lórien, and Nienna he lived up to the title of wisest of Maiar.
Where the Wise discarded the little ones he was the most aware of the Hobbits. Even Sauron in all his strategies had only heard of the Hobbits once.
Of course, someone with all this knowledge at the service of evil would be disastrous for the Free Peoples. Gandalf would be a worse Dark Lord in the sense that he understood minds better, and such an advantage would facilitate his domination.
But more than that, Gandalf as a Dark Lord would be worse because his sense of righteous would have remained:
Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained 'righteous', but self-righteous. He would have continued to rule and order things for 'good', and the benefit of his subjects according to his wisdom (which was and would have remained great).
[The draft ends here. In the margin Tolkien wrote: 'Thus while Sauron multiplied [illegible word] evil, he left "good" clearly distinguishable from it. Gandalf would have made good detestable and seem evil.'].
Imagine a world where people would be killed for not doing good, a kind of moral dictatorship but evil and hypocritical, the tyranny of good, the Heaven worse than Hell. This is something more terrible than anyone could imagine.
"...Nie wiem nawet czy chcę na ziemię zejść. Tu gdzie ciężko złapać tlen oddychamy już codziennie, ej"
"If nothing saves us from death, may love at least save us from life". Azathanai