I'm convinced 'grimdark' has something to do with delivery/execution and context. There're a lot of works that do virtually the same thing but if one is largely joyless while the other even has been slightly peppered with moments that endear the observer then much can be forgiven while some things can be jarring if done poorly.
Ultimately it's subjective but finds its way into objective reality if overused/overplayed or unbalanced and then consensus weighs in.