Why are the moves work up and cosmic power "usually useless moves" in showdown?

Question

Work up increases both special attack and attack, making it the best offensive move for mixed attackers. Cosmic power raises both special defense and defense, combine that with a healing move and you got an immovable wall. If these moves are that good, then why are they "usually useless moves"?

Comments

I don't think most mixed attackers set up, and it's probably better just to double one of your stats with Swords Dance/Nasty Plot than boost both of them at once

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also if you want good mixed setup growth is better for the chance that there is sun

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I would say for Cosmic Power, the issue is that you're dedicating 2 move slots purely to defense.  You need a way to make that dangerous, which as far as I know is limited exclusively to Stored Power sets.  You'll spend 6 turns setting up, more if you need to heal, before you can actually attack.  This gives your opponent all the time they need to switch for a Dark type (which is immune to Psychic, including Stored Power), and gives them the same amount of time to do their own setting up.  Further, if you're raising your defenses, you probably already invested your Nature and EVs into defenses as well, so your Sp Attack will probably be mediocre.

There are a few Pokemon that can make it work, like Clefable, but it's certainly not the norm.

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Mixed Pokémon are generally frail, and often have a -Defense of -Special Defense nature, so it can’t take a hit. If you have a sash, only 1+ probably isn’t enough to OHKO. As for cosmic power, I think it’s an awesome move, so I don’t understand

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Cosmic power makes the user vulnerable to toxic and whirlwind. It's not immovable at all.

Answer 1

Because Set Up Mixed Attackers aren't that common
It's better to specialize in one stat, Physical or Special. Most Mixed Attackers hold a Life Orb, or have Huge/Pure Power as an ability, instead of setting up. Plus, set up moves like Nasty Plot or Swords Dance that instantly raise SpA or At two stages are generally better.

Because Walls also specialize in one stat typically
Mixed Walls are a thing, but most people have dedicated Special or Physical walls. Having a fully Special Chansey, with max HP and SpD is a lot better than having it as a mixed wall, as an example. Mixed Walls are usually those that have equal SpD and Def stats, like Aegislash.

23 Pokemon can learn Cosmic Power; and 17 of those are Psychic, which is generally not a defensive type. The others that can learn it are either banned (Clefable) or not viable at the time of writing this (Lopunny).