So with the last update, they completely nerfed a lot of things on card gathering. I can't tell specifics, but it definitely appears that they have nerfed card gathering in Challenge, PVP, and Also on the free packs (the 400 gold cost one). Its been a little over a week since the update, so i'll give some of my personal statistics from before and after the update:

Before:

• Challenge - Stage 3 and 5 each day would net a plethora of different types of cards. Common, uncommon, and rare cards were all received from the rewards, with what looks like a better chance of uncommon and rare from challenge stage 5. Challenge is how I got my first 3 cost rare (Ghost Rider), and a multitude of uncommon and other rare cards.
• Quick PVP Match - The chances of getting a card from winning was random, but there was also a decent chance of getting an uncommon or rare. I don't play pvp that much, but in the small amount i did play before patch (around 30 wins) I would say i got about 10 cards, with about 60% of them being common, 35% being uncommon, and 5% being rare.
• 400 Gold Card Pack - I have yet to receive a rare from this, but i have received a lot of uncommon cards from it. Before the update, i'd say i would receive an uncommon for every 6-7 common cards.

After:

• Challenge - Stage 3 and 5 each day now have only net me a common card. Every single time. They've nerfed it so badly that there is no point in even doing it anymore other than the daily rewards. They took one of the things to look forward to doing every reset (in hopes that you can get a good card) into something that is now a chore.
• Quick PVP Match - The chances of getting a card from winning is still random, but now the odds of getting anything other than common appear to be drastically lower. I've done around 30 wins post update, and every single reward has been a common card.
• 400 Gold Card Pack - This one seems like it would be the least suspect as the odds are printed right on the packaging, but i digress, there is something fishy. Since the update, I have opened around 75 single card packs (3 free per day plus extras i spend my gold on since we cannot refresh gold store anymore). In those 75 single card packs I have bought, do you know how many cards other than common I have received? One (1). in 75 pulls I have received one uncommon card. I understand its still random with a 11% chance of anything other than common, but that's a far cry from 1.33% i am receiving, and others are seeing similar results.

I understand the game is still in equivocal beta, and I understand that there are odds to all of this. If it was just me with this issue, I would chalk it up to bad luck, but others are seeing similar results. They have essentially removed the rewards for doing the things they give you to do in the game (as the only way to get good cards now appears to be from the store). I usually hate those people who quickly throw up the P2W flag on an app, but I must admit, this is starting to smell like it, and the smell is foul.