While not a mono green deck, I have a Simic (blue/green) Sea Monster deck that I play this card in
If you aren't shut down by removal, your biggest problem playing Niv-Mizzet, Visionary is going to be decking yourself out
Define clear win conditions Finish games through overwhelming combat damage, repeatable burn triggers, or Dragon-enabled combos
You can constantly remove the second counter from The Eldest Reborn to force your opponents to discard a card on every one of your turns, turn Binding the Old Gods into a repeatable Maelstrom Pulse each turn, or get a steady stream of card draw or +1/+1 counters from The First Iroan Games
Not only does its damage scale into the late game, making it an insane top deck, it also has a large body with relevant abilities when you play it on curve