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Articuno's stats are hilariously bad. Just look at it in comparison to the others.
G-Arti, Zap, G-Zap, and Molt all have 125 in an offensive stat. G-Molt only has 100, but it has an ability to make up for it and it also gets Nasty Plot.
And if that wasn't bad enough, Articuno is also the slowest. And it's not even insanely slow, like it could be used in Trick Room. It's got the most awkward speed tier.
Not every Pokemon needs to be a minmaxed fast attacker, obviously, and Articuno does have the best bulk out of the birds, but it unfortunately has arguably the worst defensive type combo in the entire game.