Both Niers are RPGs after all, in terms of mechanics, and traditional RPGs are even duller (attack/defend/item/flee menu choices) but still manage to be enjoyable. From that standpoint, the bullet sponge nature of the enemies, being unable to defeat them until the player characters have leveled up enough, and backtracking is understandable.
Personally, I found the combat adequate (yes, even Nier:Gestalt) and sufficiently varied to keep things interesting.