Well i honestly think its because of the system currently
the max levels make a huge difference.
Hero mode in Vindictus is only for level 50, so 15 and under dungeons would be easy.
However abyss is unlocked by a good rank on master, and the total level is 25 ( if i remember) so 15 and under dungeons would be harder in my opinion.
Im sure if they had a cap for level 50 only in dragon nest 15 and under dungeons would be easy too...