Gigantamax Pokemon

Charizard: Cannot set up sun and still suffers from the crippling Fire/Flying dual type, but can possibly sweep with a STAB Flying max move.

Butterfree: Absolute shit mon with 33.3% chance to put you to sleep vs 75% from just using Sleep Powder.

Pikachu: Nobody is using this thing either way.

Meowth: Entire purpose is for grinding and also same as above.

Machamp: Trades Atk boost spamming for a crit raise, when it looses the secondary effect of moves like Cross Chop to even put it to good use.

Gengar: Trapping > Def drop, but just switch lol, can still possibly sweep with a STAB Poison max move, though.

Kingler: Trades setting up rain for inflicting -2 Spd, becoming useless for rain teams and inferior overall.

Lapras: Sets up Aurora Veil over the ability to set up hail, not being a totally bad trade off, but Lapras is too much of a wasted slot just for that attack alone, can still set up rain with STAB though.

Eevee: Same explanation as Pikachu.

Snorlax: Just send out Dragapult lol, but no, this is arguably one of the best Gigantamax if left unchecked, I will admit.

Garbodor: Trades the ability to sweep with a STAB Poison max move for merely inflicting poison, and even then, it only has 60 base Sp. Atk.

Melmetal: Literally just switch, and it's already an Ubers mon anyways.

Corviknight: It's G-Max move is inferior to the normal Flying Max move and can no longer speed sweep with it, defog is useful but better on a different Pokemon.

Orbeetle: Gravity is a meme and it loses out on setting up Psychic Terrain, and it's a shit mon anyways.

Drednaw: Cannot set up rain to benefit it's own hidden ability and support a rain team, but can still set up a sandstorm with a STAB Rock Max move though.

Coalossal: Cannot set up sandstorm any longer, but can still set up sun with a STAB Fire Max move, but it'll still take a nasty ground or water type attack before it can even attack though.

Flapple/Appletun: Loses out on setting up Grassy Terrain for lowering the foe's accuracy, which would be helpful for Flapple since it has Hustle, but all Max moves land no matter what anyways, making the debuff useless.

Sandaconda: Neither Ground Max moves are impressive in the slightest, as well as the Pokemon itself.

Toxtricity A/L: Cannot set up Electric Terrain any longer, but can still possibly sweep with a STAB Poison Max move.

Centiskorch: Loses out on setting up sun and gets a completely inferior attack instead.

Hatterene: Cannot set up Misty Terrain but can still use a STAB Psychic Max move to set up Psychic terrain, but it's speed is so low, nullifying priority will change nothing anyways.

Grimmsnarl: Trades a Sp. Def drop for Yawn, which are both removed by just switching, but it can still set up Misty Terrain with a STAB Fairy max move.

Alcremie: Trades setting up Misty Terrain for healing the entire party by 1/6th their full HP, which does very little to even make a difference.

Copperajah: Setting up steel type spikes is one of the best G-Max moves and a huge upgrade over the normal Steel Max move, and it's one of the Gigantamax Pokemon that could be argued against.

Duraladon: It's G-Max move could potentially permanently cripple one of your foe's Pokemon, but it has nothing going for it in terms of setting up weather, terrains, or boosting offensive stats without using weaker non-STAB attacks.