Could it be? Is it really good now?
The last few sets have been pretty nice for all-in aggressive strategies. Cards like Become Immense and Temur Battle Rage have already shown up in Modern and Standard. Monastery Swiftspear and Abbot of Keral Keep are headlining red decks in both formats as well.
So, about those Nivmagus Elementals...
We might finally have enough things to make a dedicated Nivmagus Elemental deck a thing. We have sixteen red creatures that all want you to play a bunch of spells, so we're redundant in that aspect. Temur Battle Rage and Ground Rift get you through blockers. Lightning Bolt can also help clear the way or get in the last few points of damage.
Being able to play Vines of Vastwood is probably the real clincher. Being able to protect your creature from Abrupt Decay is a big deal, as is having a maindeck card that can counter a Splinter Twin. Additionally, it's a pump spell when you need it to be.
The toughest matchups were always B/G/X because of their discard and removal. They could often pick you apart before you got anything going. However, this version of the deck likely plays out similarly to a Burn deck, getting in chip shots when it can. If that's the case, maybe Jund and its ilk aren't that difficult after all. Even if they are, we've got four Blood Moons lurking in our sideboard.