There is at least one Dartmoor cache worthy of its 5/5 and that is Muddypuddles GC28Q1K, Great Scientists: Albert Einstein. At the time I rated it as 15/15 but that was mean’t to be a joke!
Normally if I can climb it at my age its terrain rating cannot be a 5. I have done a 5/5 which I would have rated 2.5/2.5 the only equipment needed being a torch and it could be found without even looking at the puzzle to work out where to go.
On a serious note, I agree with Dave’s comments on this subject except for his interpretation of the length of time it might take to find caches of the various ratings. I am too impatient to look for much over 15 mins.