Furthermore, there is only one scoring system for a lot of different things involved in perceived difficulty: conceptual difficulty (like hard math problem with a simple formula solution); algorithm difficulty; programming notions involved in resolution (the theoretical kyu ranking)… With the last two relying a lot on the language! Then, even if focusing on programming notions, ranking depends mostly on the first language used for the kata. I guess most people are feeling difficulty mainly according to algorithm and/or conceptual difficulty. But codewars is about programming :)

Love it

Old katas are (generally) overrated, I think the rules were specified later, new ones tend to be ranked lower than them because power-users try to follow them now.
Some of them were downranking katas (to level the field) but that stressed the server and can't be done anymore.

Sorry, but if these rules were respected, the large majority of the kata should have level 7.
For example: 6kyu: https://www.codewars.com/kata/find-the-nexus-of-the-codewars-universe/javascript
5kyu: https://www.codewars.com/kata/directions-reduction/javascript
Also i don't see any of those rules in those katas :P

5 kyu kata represent a novice programming level. At this level the kata are similar to 6 kyu but more challenging. It includes:

Complex language features that require mature OOP/Functional concepts
Advanced OOP/Functional concepts
Complex Design Patterns
Advanced regular expression usage

I don't see any of those here.