Ah, this one...I remember going through the LN a bit.
The things that turned me off were:
1. Needless levels of "bad luck / karma" against the MC. Everything that can go bad for him, absolutely will go bad for him. That had to happen in order to turn the MC into super edge boy.
2. Stupid game mechanics. Now, I don't hate game-mechanics, but I sure hated them in the LN. It's one of those systems in which if you kill a goblin with a fireball, you level up your ability to swim. It was also completely and utterly unnecessary, as the MC already had a control over dark-magic by that point. It was a completely redundant insert.
So all in all, you have an EDGE BOY EXTREME, and you have unnecessary game mechanics.
P.S: That's my opinion about the LN. I have no idea how the manga goes, just wanted to drop my opinion in case anyone like-minded wanted to check out the LN.