Its shit because... it is. There's no suspense, no danger that the MC will lose when he is shown overwhelming every situation regardless of the circumstances. Then, the plot is so one dimensional, its sad: portals open and special humans, hunters, fight back against the invasion. Now you might argue that there is some mystery since we don't know the purpose of these gates, but does it matter? The answer is no.
In fact, I would argue this is quality in the same vein as DBZ; entertaining enough to keep people watching/reading, but in no way can say that stands at the top in any way, shape or form.