The MC got continuously summoned to different worlds to the point even their race was affected, it's just absurd. It's not like the guy ended up having their soul disassembled and reassembled into something not human. The goddess clearly stated it was their soul getting pulled to the other world while the same vessel seems to be prepared during transport for the summoning. So how they ended up a demon is unknown. The guy can only use their full power during the day, is apparently a "demon" yet can get healed and don't suffer severe damage. There was an item introduced that can't be cut, buta few chapters later, was severed by a mythril sword. There was a lot of things that don't make sense that all I can say for those who want to read it was "turn off your brain."