After a near death accident leaves a man with the ability to see behind the veil, he soon finds his real mission is to stop a unique werewolf from tearing apart the veil and destroy reality.

After an accident leaves [John] clinging between life and death, he's mysteriously given special abilities to fight devious creatures behind the veil in order to save himself - and everyone else.

My two cents. Story sounds very interesting.

Word economy is your saving grace - as it is now, this logline is confusing and vague.

The accident seems less related than it should be, he could get the powers whether or not he's dying so why bother with it in the logline? Best to make the inciting incident directly connected to his goal.

For example:
After a self centered lawyer arrives in limbo, he's given the option to fight a mythical creature trying to destroy the veil between this world and the next or go to hell.

This way you've made the goal a direct consequence of the inciting incident and at the same time, you've given him a clear stake. I also thought it would be nice to give him a flaw that he would have to overcome before he achieves his big 'A' plot goal.

Let's look at the pieces:

EVENT: Super-powers bestowed on protagonist's deathbed.
ACTION: (with the most, although vague, stakes) Stop creatures trying to destroy the veil between worlds. (Definitely need to focus and refine this. It's not even clear what that means, or why it's important).
FLAWED PROTAGONIST: a man. (Even if we describe him as an UNDEAD man ... you can do better than that, right? What's the character's flaw, and what is their function in the world at the end of act 1?)
ANTAGONIST: Unclear. General mythical creatures.
STAKES: The protagonist's soul? Though it's unclear how that is at stake. Because I've seen stories like this before, I know that 'destroying the veil between worlds' sounds like a generally bad thing ... but I don't know whether that means that monsters can roam freely on the streets of victorian London, or whether it means the collapse of the universe and total oblivion. So this needs to be made MUCH clearer.
DEADLINE/TIMECLOCK: Not clear. Maybe not necessary.

"After he's bestowed with super-powers on his deathbed, a cowardly garbageman must stop the King of the Goblins from merging the fantasy world with our own -- and thereby unleashing hell on Earth."

You'd need to indicate WHY these superpowers that he receives now make it possible for him to put up a fight from whatever the actual antagonist of your film is. Like ... get specific with them. What he can do is going to give me an indication of the tone of your film (Is this Underworld or Men In Black), and basically form the hook that helps sell your screenplay.

Not knowing the actual flaw in your character makes this hard to judge, but make sure that it conveys the irony in the premise. (Who would be LEAST equipped, emotionally and physically, to stop the events that are occurring).

Again, the goal and stakes of your logline could be clearer.