Do unlockables run out?
If you unlock enough cards to reach rank X on a weapon or rank V on a weapon mod, it is removed from the pool, and it is no longer possible to find it.
This also works with consumable supply increases (up to +5), and character respec cards (if you have 3 unused respec cards in your inventory, you can't get any more).
What doesn't get removed from the pool are the character cards, because even after unlocking all of the appearence options for a given character, you still benefit from the class XP (more xp from rarer packs!) each time you find a class character card.