FACT. Of the 14 instances of Promotion (you chose to illustrate your point) from the National League North to the National League, on 12 occasions they have been relegated back down to National League North.
MY OPINION BASED UPON THAT FACT. With such a high regualrity of promoted teams being quickly relegated from whence they came; It is reasonable to take the position that the (mainly) Full-Time Professional National League presents a considerably more difficult challenge than the (mainly) Part-Time Semi-Professional Regional League.
Do you agree with the above Fact?
Yes? Good let's move on
No? no problem, disavow it with evidence, I am all ears.