Omnitype doesn't work as advertised.

[Trevor Frueh]

When using or against Giratina with omnitype, some of the type matchups are not how they should be against a pokemon with all types. Giratina becomes 4x weak to Rock, 2x weak to Flying, Fairy and Water, neutral to Dark, and immune to Grass. Giratina should be 2x weak to Rock, neutral to Flying, Fairy, and Water, 2x resistant to Dark, and 16x resistant to Grass, which is practically immune, but still incorrect. All of the types that Giratina should be immune to work normally, and the remaining types (Fire, Ice, Steel, and Bug), have not been tested. This is as of version 1.2.2

[Frelus]

Last time I tested, it was also weak to Ice, but that might have been fixed. It is still (As of 1.2.3.6) at least 2x weak to Fairy, though.

[Frelus]

Still weak to flying in 1.2.3.6 as well.

[Frelus]

Also weak to water.

[Alberto Hensley]

I have tested Fire. I have found out that Primal Giratina is weak to fire type moves. This occurred when fighting the double battles against Taen, with Gail, with my Primal Giratina as I switched out my burned Mega-Metagross (Spider) after it got burned via Flare Blitz and set up Sticky Webs. Talonflame used Flare Blitz on my slot as was said as super-effective. (As of 1.2.3).

[Jack Dod]

Weak to fairy. Hit with Play Rough by an Gail’s Azumarill.