The golden Holy Grail radiated dazzling light. Bathed in that glow, everyone felt, for some inexplicable reason, that this cup could grant every wish they had!
Of course, Shiro knew that was only an illusion. Though the Holy Grail was called an all-purpose liver-bursting machine—a wish granter—it was not truly omnipotent. The wishes it could grant had extremely strict requirements. Rather than calling it wish-making, it would be better to say it instantly completed a certain process.
Yes, though the Holy Grail was called an omnipotent wishing machine, at its core, it was still magecraft, even if it was large-scale magecraft! (Note: the Holy Grail is only a derivative of the Third Magic, not the Third Magic itself.)
Therefore, this wish had strict requirements. You had to know the method to realize that wish, and that method had to actually exist.
For example, if the wish was also to save the world, Amakusa Shirou Tokisada's wish in the Great Holy Grail War could be realized, because he had a method to make it happen—the materialization of all humanity's souls!
Setting aside whether that method was correct or not, first of all, he truly did have such a method, so a proper Holy Grail would grant that wish. As for someone like Kiritsugu, though he likewise hoped for world peace, his heart was lost. He did not know how to do it. Killing the few to save the many was a method he had abandoned. Naturally, a proper Holy Grail would not answer his wish.
Of course, if proper ones existed, improper ones naturally existed too. An improper Holy Grail was truly "omnipotent"!
It could grant any wish for you. It was just that the result might not be very satisfying.
Take Kiritsugu as an example again. If he wished for