Ultimately the biggest issue was stripping the film of all the interesting bits of the manga, and focusing on the action sequences. The action scenes are shot well, but hold no impact considering it ignores any substantial buildup. The characters are just there, simply set pieces purely to exist as proponents of high octane action scenes. Changing Kei's character from a borderline sociopath to a generic protagonist who just wants to protect his sister was the nail on the coffin, it truly is boring and lifeless as hell.

I'm not complaining about it deviating from the manga exactly, just that it really got its priorities wrong. What makes Ajin interesting are the characters and how they interact with each other, the fights are enjoyable precisely because of that. When you take that away, it's just... Mindless action.
Nice cinematography, acting, and the special effects aren't too cheesy. That's about it.