Games like Dark Souls and Bloodborne have conditioned modern day players to expect a challenge. There is a certain school of thought among players that believes if a game is not as hard as all hell, it's either (a) not worth playing, or (b) a Nintendo game.
That said, even the most masochistic player out there will tell you that bosses who can kill you in literally one hit simply have to go.
Take Final Fantasy VII's Tonberry, for example.
Here you have an enemy who, whilst small in stature and arguably pretty adorable to behold, can decimate your entire party with as much effort as casually swatting away a pesky fly. He is truly a nightmare, and could definitely be said to be somewhat unfair, despite the fact that he can - rather obviously - be beaten in the end.
There is, however, a subtle yet crucial difference between one-hit-kill bosses, and enemies in RPGs such as the Xenoblade series, who can kill you easily since they are of a much higher level and are not meant to be tackled early in the game.
These are the exception though, and the truly unfair, insta-death bosses can - quite frankly - do one.