I'd say being able to play with an undersized deck makes it too easy, because you can take just one card that grants double-attack or taunt and etc. and chain them with certainty onto a supermonster, and have the rest of your cards be maxmagic and walls, with a leader-archer or something to make up for inconsistency. I found a 15 card deck was the strongest. I don't think it's a bad thing, since you need to worry about boardclearing cards and such, and it being singleplayer the balance isn't that important.
Daffodils as a leader card is crazy though, it just turns into endless board-wipes at 10, it sticks out too much for countering it to be fun unless it was a set challenge-match where you play one round knowing what's coming. And it's exploitable in the worst ways, where I'm not sure there's any counter to a healwall Daffodils deck-out strategy, except the passive agony of participating in that. You go minus for clearing the field with a card, but in a game where monsters can't attack on the first turn being able to click board-wipe each turn is too much.
I'd also really like the card-packs to be location specific, if they aren't already and I just got unlucky. I went to get a card I wanted from the area I thought gave it out but then got completely unrelated cards.
And I think it's best not to increase the number of rounds with the difficulty level, since the enemy health is already higher so it combines to make clearing it take more than 4 times longer or something at level 3.