Deck slots I can get behind. Card slots seems a pay to win strategy.
Deck slots is 100% a convenience issue but card slots directly effects the power of one deck over another. I would suggest that instead the decks simply have a set number of total card slots and we get to determine how many copies of a card appear on that deck up to four. That way if I want to run only two copies of one card and two copies of another for a total of 11 unique cards, I can, but there are half as many of those two.