Why people keep saying that Iron Head/Poison Jab Garchomp fuck up fairies?

>Sylveon can take a Poison Jab/Iron Head then OHKO back
>Timid Max EV Mega Gardevoir probably outspeed Adamant Garchomp
>Xerneas, same as Sylveon
>Azumarill can take a Poison Jab then OHKO back with Play Rough, assuming it hit AND that Garchomp has no defense/HP EVs. Play Rough is not guaranteed to OHKO MEGA Garchomp, though

Those fairies are like the most common ones out there.

That's the same for most Fighting/Dragon type Pokemon who can use Poison Jab and/or Iron Head.

Anyway discuss Pokemon that can't properly deal with its type weakness even with coverage, like Psychic Porygon2.