I agree with Mike N, often saying there is nothing wrong with a hard hole once in a while. Too often, and a course would be no fun to play, but once per course, or per nine, I think most golfers like challenging the difficult hole. And, they like one or two holes to talk about in the bar afterwards......and they usually talk about their birdies, and when about holes, its either hard, beautiful or unusual they talk about, depending on their golf personality.
To me, conceptually, a shot can be too hard, like, BT 14, if the target zones just can't be hit with a slightly above average shot from a good player, there is no real bail out (or as Ian says, alternate strategy for par) to offer a reasonable chance at bogey, and the penalty is typically repetitive strokes, which is the case from way below the hole at BT 14. (from my one play and others experience)
A hole can be too hard if it contains hard tee shot, hard approach, and hard putting. It seems as if conceptually, there should be some reward for a tee shot that hits a narrow fw, or the right part of a wide one, if it has a great hazard. Otherwise, where is the strategy?
And, if you have a hole with hard tee shot and approach (say to a small green) then there surely ought to be an easier putt as some kind of reward, rather than just another beat down.
A lot of things affect "too hard" but a lot of it is target size, relative to length, wind, etc. As to one hole that I recall thinking was way too tough, PV No. 5 came quickly too mind - FW wood or ling iron to a perched green with fall away to slice side and bunker right where you might land short to get on green? I know I didn't have a shot to play it.....