>>34661805It comes from her typing; it gives her a 4x resistance to Fairy, a trait shared only with Heatran (she also has 4x resistance to Grass and Bug too, for the record). Trouble is, Salazzle's defenses are terrible, she's weak to Psychic (and almost EVERY Fairy Pokemon knows at least one Psychic move) and she has a 4x weakness to Ground. She can handle resisted moves okay, but a hit from a neutral STAB move can bring her down easily. On top of that, she gets no coverage for any of her weaknesses (so you basically have to pray for a Hidden Power that's decent), and the only recovery move she can learn besides Rest is Leech Life, a physical Bug move.
And as far as her ability goes, it only allows her to poison Steel and Poison types, but it doesn't help Steel types still being immune to Poison type attacks. In other words, Salazzle can use Poison Gas, Toxic, and Fling (with a Toxic Barb or Toxic Orb) to poison them, letting her put a timer on bulkier Steel enemies or break through Sturdy, but she won't likely survive the fight long enough to fully capitalize.
Now, if she had a signature Poison type move that can poison AND hit Steel types for super effective damage (like a Poison version of Freeze Dry), and also got Mega Drain or Giga Drain, then she'd become a far more legitimate threat, maybe even be in UU.