The limiting factor on competitive deck building is Rare / Mythic Rare availability. Lower rarity cards simply aren’t as powerful, and the availability is high enough for them that they’re almost never an issue.
As you open random card packs, you are guaranteed to open one Rare or Mythic Rare per pack. Arena offers duplicate protection - you will never receive a Rare / Mythic Rare that you already have 4 copies of in a pack. Thus if you already have a complete-but-for-one-card collection, you’re guaranteed to receive that one in your next pack. It also means that there is a maximum number of packs you could need to open to have a complete set of Rare / Mythic Rare cards.
In addition, sometimes your Rare (or Mythic Rare) is replaced with a Wildcard of the same rarity. These wildcards can be crafted 1:1 into any card of the same rarity. This does not increase the rate at which you grow your collection, but does increase the speed at which you can grow it in a particular direction. The drop rate of these is tuned with the intent of allowing a typical free-to-play user to obtain one competitive level deck per card set.
As you open random card packs, you are guaranteed to open one Rare or Mythic Rare per pack. Arena offers duplicate protection - you will never receive a Rare / Mythic Rare that you already have 4 copies of in a pack. Thus if you already have a complete-but-for-one-card collection, you’re guaranteed to receive that one in your next pack. It also means that there is a maximum number of packs you could need to open to have a complete set of Rare / Mythic Rare cards.
In addition, sometimes your Rare (or Mythic Rare) is replaced with a Wildcard of the same rarity. These wildcards can be crafted 1:1 into any card of the same rarity. This does not increase the rate at which you grow your collection, but does increase the speed at which you can grow it in a particular direction. The drop rate of these is tuned with the intent of allowing a typical free-to-play user to obtain one competitive level deck per card set.