It definitely makes certain games easier, particularly those a single, unchangeable difficulty. And also particularly with games that dont have good aiming controls on a gamepad in the first place, like GTA V(I bet Uncharted/The Last of Us would be much easier with keyboard/mouse).
But with others, it can sometimes be a call to throw the difficulty slider up a notch to balance it out more.
And some games are indeed balanced for keyboard/mouse controls, but I'm assuming we're talking about multiplatform games for the most part?