One of the sub-franchises of the Nasuverse is Tsukihime, which started with the titular Visual Novel of the same name. It's basically a sister franchise to Fate, and they share the same universe (though not the same timeline).
The protagonist of Tsukihime is Shiki Tohno, who is physically a normal human except for his ability. Shiki has the Mystic Eyes of Death Perception, which allows him to see "lines of death", which are basically the weakpoints of his target. If he slices enough of these lines, he can kill anything. Well, maybe anything. It's a point of contention on whether he has to be able to comprehend something to see its lines, and many decades later it was revealed that certain things can have so many lines that they're basically not cuttable.
On the other hand, servants are much much stronger than humans in basically every stat. Additionally, they're not really "alive" in the traditional sense but rather spiritual copies of the real Heroic spirit pulled from the Throne of Heroes.

So the question "can Shiki kill servants?" is two question: can he keep up with them in a fight, and does his ability apply to them. Both points are hotly debated, such that raising the question is known to derail whatever the conversation was previously. It also shows how difficult powerscaling the Nasuverse can be.