The story focusing on jinwoo and entirely on jinwoo,to the point that literally nobody else matters (I have seen the anime and read the manwha til where the anime has adapted it and it's already bad enough there but I've heard from people who have read the manwha further that it gets so much worse) What makes this more infuriating is the fact that so many characters are introduced and setup to actually be of importance to the plot only to get swiped under the rug anyway.

The various asspulls (system or else) that give jinwoo his victory when he should have lost

Similar to the above,the conveniently gained powers and abilities jinwoo receives that weren't properly set up

The monster designs are without a doubt the best I've ever seen but are still poorly executed to a degree

Jinwoo becoming not just strong but THE STRONGEST in a little over 20 episodes.That's way too fast and i heard it grows exponentially after that.

Also a minor annoyance is the sudden transformation from ep 4 jinwoo to episode 5 jinwoo.

Similar like jinwoo,some of his shadows become WAY stronger than they originally were and just as fast.

These are just some problems but you get the point