>>33902855I mean, that's swell and all, but there are some nigh-objectively bad Legendary Pokémon in competitive play.
Type Null and Silvally: underwhelming movepool and lackluster offensive stats. Abilities are also rather constraining.
Phione: Terrible stats all around and doesn't even get Tail Glow like it's parent Manaphy.
Regigigas: Horrible ability, if not the worst. Can get worn down very easily and even lacks some TM moves that most other Pokémon can learn like Protect.
Cosmog and Cosmoem: Literal Dex fillers with no means of attack or recovery.
Guzzlord: Slow and has "Big HP and paper defenses" syndrome, meaning it can't tank nearly as well as it alludes to. Can be rather problematic with Trick Room in play however, as it has workable offenses.
Uxie: Pretty much only exists to be support, as it's too weak to take on anything by itself. Can easily be stopped by Taunt.
Mespirit: While versatile, it's slow and very susceptible to revenge killing. Also very prone to being hit by Pursuit.
Regice and Regirock: Poor defense typings and lack of recovery really hurts their roles as bulky attackers.