Advanced Packs only drop uncommon weapon mods after all other uncommons are maxed. 6 chars/40 guns:

They will drop, but it's definitely scaled/weighed to unlock characters/weapons first.

With basic and advanced packs in general, you have a lot less chance of getting a weapon mod. I wasn't able to max out my common weapon mods until I had maxed all of my weapons and characters; therefore forcing them to unlock.  I believe all of my common mods were initially unlocked by uncommon packs.

After the recent patch and addition of XX character's, I spent the majority of credits on basic packs. I think I got a total of 2-3 uncommon items that WEREN'T consumables. Likely due to the pity timer. So the guarantee for an char/weapon/mod only exists with Expert Packs and above.

Spam basic packs all day for example, and you'll rarely even see anything outside of consumables. I don't have my uncommon's close to X on anything, besides characters.   Most of my uncommon weapon mods are from Expert Packs.

So likely, I won't be able to unlock all of my uncommon mods, until I level all of my uncommon characters to level XX, and weapons to X.   

Maybe someone else can clarify how items are "weighed"