I'm not understanding the criticism of this card at all.
1) Duskbringer is an INCREDIBLY powerful card for an incredibly specific deck archetype for a totally different class that does not have very much follow up to it (particularly in the Dragon Priest archetype). The comparison isn't really fair. This is a mage card, Mage has a ton of ways to draw this card and a ton of ways to follow up on a 3 damage AoE into a 3/3 that priest does not.
2) Baron Geddon isn't as strong as this. It has better stats, but it's a legendary. You can only run one. It's also a significantly more expensive card (turn 6 vs. turn 7) which really matters against aggro and swarm decks and 3 damage, particularly in regards to an AoE effect is a lot more than 2 damage is. You clear a lot of hunter boards that Geddon doesn't. You clear a lot of Odd Paladin boards that Geddon doesn't. You clear a lot of Murloc boards Geddon doesn't.
3) It's an elemental. Not only does that mean that you get the benefit of Frost Lich Jaina (which is somewhat relevant) you can also follow it up with a blazecaller or a tol'vir warden. It can be drawn with Book of Spectres as well as copied with Arugal.
4) It provides you with a 3/3 body unlike Flame Strike and does more damage than Blizzard as well, it's more reliable than Dragon's Fury and doesn't require the same specific deck that Dragon's Fury does. And again, it benefits from Book of Spectres and creates Elemental chains. I don't think you're running this in big spell, and I doubt you're running this in traditional control mage. This is totally going into minion and elemental mage decks.
5) The idea that this gets worse next rotation when we haven't even seen the full breadth of THIS set yet is so irrelevant. We have no idea what the 2019 meta is going to look like. This could be a totally essential card against all the 3-health turn 5 board clears that everyone is playing because they introduced a neutral legendary that summons those. It's completely academic. And I don't think this cards NEEDS Jaina to be good anyway.
6) The only complaint I think is somewhat fair is the idea that this damages whatever elemental you played last in the chain. Typically that's going to either be: Mountain Giant, Bonfire Elemental or a Fire Fly. I think typically you're going to specifically activate this with a fire fly, though. Or maybe a glacial shard or something. So destroying that is mostly irrelevant. I think if you're going that hard into AoE plays there's going to be a reason for it and you'll absolutely be doing more damage to your opponent's board than your own.
For #6 though, you can trade in the elemental you used as an activator before playing this. Its useful for taking out a minion with more than 3 health.
Even after Jaina rotates, this will see play in a mid-control style elemental deck that seems fun.
But yeah, when you can use an AoE spell, you will probably do that instead.
Because it comes on the turn immediately after the Hunter spellstone and completely clears the board AND puts a 3/3 in play, instead of not fully clearing the board and putting nothing in play (in the case of Blizzard), or coming an entire turn later (in the case of Flamestrike). This argument is like saying "I don't understand why you'd run Duskbreaker when Holy Nova exists."