This thread explains pretty much everything: 

https://www.reddit.com/r/Paladins/comments/7glgis/why_cards_unbound_is_an_absolute_nono/

I'll just summarise the points.

1- This system WILL be Pay 2 Win.

2- You need too much grind.

3- If you want to play casuals, they'll be extremely unfair to people who can't grind a lot (or can't spend money on Paladins)

4- You can't practice stuff for competitive.

5- There will be less customization for the loadouts.

6- People with high level cards will have a ridicolous advantage, even if it's just casual (Not funny)

7- The balance is currently horrible (that's not a main problem anyway)

8- Level 3 cards are not enough for ranked. You won't be able to achieve some stuff that was really important in your playstyle.

9- Mobility is nerfed once again.