The game’s UX may be a bit misleading. When you play a card that costs 1 HP, the ability on a card that costs 2 HP is doubled and turns red. However, the red color gives the feeling that the card cannot be played, even though it actually means the effect is doubled. So the actual meaning and the player’s feeling may be opposite.
Also, this doubling mechanic feels very suitable for a longer game. After developing your build in the early game, it would be very satisfying to deal huge damage to enemies whose HP and damage also scale up. Since the current game has relatively few levels, the mechanic doesn’t fully show its potential yet, and it also makes the early game feel easier.