Why didn't Hachiman level up despite defeating a Minotaur in a single blow and possibly saving his students from troubles in the past while being the same level as him? Because his fighting style is too effective, ridding battles from the threat needed to force him to level up as stated by Freya about the fight against the Goliath.
Why is Kaori capable of talking while planning or doing other stuff, despite the single-minded nature of her skill? Because it allows her to focus on a single task, not action.
Fridge Horror
Kaori's appearance in Under Resort opens up the possibility of more people being sent into the Danmachi world and appearing in the Safe Zones after the Boss Floors in the Dungeon. This by itself can get horrible enough taking in account that by the time Hachiman gets to level 2 Kaori has gotten up to level 4, meaning she had to combat either the monsters below Riviria or those upwards, in either case she was forced to fight monsters capable of effortlessly killing adventurers twice if no more powerful than her in order to survive those two years they have been trapped in Orario.
If other people were indeed sent into those Safe Zones yet they have not been found out despite familias like Loki's doing expeditions from time to time, it might mean they suffered a horrible death either by the Floor Bosses or regular monsters. Take into account that one of them might as well had been Yukino, or Shizuka, or, in the worst case scenario (for Hachiman), Komachi.
Hachiman's fear of being just delusional is a subject usually not brought up by other Isekai stories, however the very real possibility of simply having gone insane and what he thinks is real being simply delusions would take a toll on anyone because being alone in another world means there's no plausable way to confirm their memories of their previous world are real and in turn, there's no way to confirm what he is seeing as "Orario" is not the product of hallucinations.
Hachiman's own skill might seem like a completely broken one but it has backfired more than once. This could potentially bring an And I Must Scream situation in which his own mind is the constant source of torture to him.
