Difficulty ratings are pointless. What one person finds easy another finds hard and vice versa. I solved most scientist caches (Robert Hooke is in my opinion by far the most difficult one of the lot) in a few minutes and struggle to make anything of far lower rated ones – especially cryptic ones. And with these new challenge style caches the difficulty rating makes even less sense.
Terrain ratings are a bit more straightforward although I think I disagree with Einstein being a 5 as we didn’t really need any special equipment for it (actually that goes for the Code breaker V: Cryptology as well) .
As for Dave’s comments that the last few yards don’t matter. Something hidden 40 ft up a tree next to the road should be a terrain 1.5?