Scizor - With regards to scizor, its usefulness is more a commentary about how good uturn is than how good the pokemon is. It actually has many reliable answers that just cant do anything to it because uturning is the most inconsequential move in the game. Still though, uturn, knock off, scald, and all of these broken moves have given rise to so much counterplay that i frankly think its laughable to call scizor unhealthy when youre actually just admitting that you dont know how to play around uturn strats. I honestly think you have to have a very 1 dimensional team to not be able to beat sciz.
Latias - Trying to hurt mega latias is like trying to punch in a dream. It just doesnt matter how strong your move is, it ends up barely scratching it. A special attacker will literally never beat latias 1v1, so immediately half the tier is meaningless against it. Even the strongest physical attackers have a hard time 2hkoing it with its 120 defense maxed out.
Altaria - On paper this thing seems even more broken than latias. I have not seen much evidence that says otherwise. Even if youre not running dragon dance, a defensive cotton guard set is unkillable, and ultimately just points to the fact that this thing has about as many, if not more viable sets than mega latias while being similar in brokenness to it in all other categories. I would really like to see more of it in action, but cannot fathom it remaining UU. This one falls under the category of "has many counters but actually no counters if you think about it"
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I would like to go back to scizor, because I think it is an interesting talking point about what makes a metagame fun. I for a long time hated pursuit, because i though it to be inherently unfair, until realising how 1 dimensional the game would be without it. Part of the fun of pokemon is making predictions to work around moves like that, and building teams which take advantage of this strategy. I ran metagross in rain once upon a time, and after realising metagross is easily trapped, I put it with seismitoed to come in for free and sweep on a muk. It's these sorts of things that I find fun in the metagame, and banning them or the pokemon that use them would make the meta way too linear, and I think the same of uturn, knock off, scald, and all that.