This works on any type of coating, flames and poison damage too.
It's definitely a powerful buff to GWS, which it admittedly does need. Not sure how out of hands this can get in a game like this though, because stack enough dmg dice and it will likely get pretty absurd.
How does Great Weapon Fighting need a buff in your mind? It's already benefitted by the most powerful weapons, made stronger by the most powerful weapon feat (Great Weapon Master), made even stronger by Larian giving it the best/most Weapon Actions and the best magic enchantments. It needs serious nerfs more than most.
BG3 isn't D&D as much as a Larian meme at this point, and you don't do character building around 5e nearly as much as you make builds around their broken implementations and overpowered items.
