Dick Daley,
Thanks for your thoughtful input. I really started the thread to ask a question. I didn't have an answer in mind and I am not sure I have played a hole that is "too hard", even the example of #14 at Pine Valley which I last played about twenty years ago as, I think, about a 12 HCP. The yardage on that occasion was about 170. I debated clubs and think I went with a 7 iron thinking the hole would play more like 150-160 due to elevation. The main thing was to just not be intimidated - a common feature of holes at Pine Valley - and just hit a shot I was capable of hitting. And I did, making a par I think.
Now, that wasn't 215 yards, of course. Therein, lies part of the problem coming up with a definition. The hole is pass/fail like Pat suggests, but ?I could pass as a 12 from 170 yards. So, maybe it is the 215 yard tee that is "too hard", not the hole.
As for the question of one golf hole verses an entire course, I'm still inclined to think those are two different things. Both face a definition problem.
By the way, I agree with your suggestion of a cut off at 20 HCP as someone who experienced playing Pine Valley with a 36 guy. It was brutal for both the group and the caddy. The amazing thing was the caddy politely avoided picking up a ball until #18 just because the guy was a guest of Club Chairman Ernie Ransome.