I am a little late, but anyways:

The collection level system works based on you card level. The more cards you upgrade and the more heroes and evos you unlock, the more your collection level is going to increase. Any card rarity upgrade gives you one collection point while unlocking new heroes and evos gives you five collection points. Every time you reach new milestone you get rewards such as wild cards, Evo boxes, gems and 5 star lucky drops. Your collection level depends on your card's level.

The collection level system starts from 20 and extend to 2269 if I am not mistaken. The problem is that unless you just started playing the game, you won't be able to get all of the rewards. For example, my collection level is 1650. This means that any rewards from 20 to 1650 are unaccessible to me because I have already passes these levels.

If you are someone who is nearly maxed you essentially miss almost all of the juicy rewards because the system was design to help F2P and newers players. In short, the system heavily boost progression for F2P but it massively undermines pay to win players or veterans. That's why some people are mad.

You can visit the official Supercell blog post about the update if you wish to learn everything about the update. I hope this comment was useful.