All of their free games do have "loot crates" system, but in Dota 2 and CS:GO, it's only cosmetics. TF2, you can also get weapons and in theory all weapons are "balanced", though in practice perfect balance doesn't exist, and also having more options available + being able to switch loadout during a match can be more OP. That being said, the weapons themselves aren't very rare, and you can get most of them fairly quickly, or craft the ones you need. Hats are more rare but those are again just cosmetic.