I saved up 50 packs from Daily Arena and Warzone wins so I could open them all at once after the Memories of Reach update dropped. I had all my Common, Uncommon, and Rare permanent items unlocked at that point, since I waited I actually had some lower rarity permanent items in my req pool to unlock again.
Out of 50 packs 7 gave me permanent unlocks, the rest gave me 100-200 req points. (If I recall correctly it was all of the common items, 2 emblems, new attachment on AR & BR, Brute Plasma Rifle, Brutalist Stance, and then one of the azure dream skins)
So it appears to me they have a moderately lower chance than a bronze pack to unlock a permanent item. I cannot recall if the skin came last, indicating I had exhausted everything of common level before it would unlock something of rare level, so I am not sure if it is completely random, or if it will always pull from lowest pool first.
(Edit: To clarify I am talking about permanent unlocks from daily packs (certifications/cosmetics/loadouts/armor mods)
