I think one difference between card tokens and action/coin tokens is that you want the extra card right now far more often.
It's easy to have no use for an extra coin right now... it's common to want to buy something that costs less than what you produced. Whether that's because you produced more than
this turn when you're into the Province-buying part of the game; or you simply produced
buy would rather have a
cost than a Gold.
Similarly, with actions; it's plenty likely that you produce more actions than you can play; based on not having terminals in your hand.
But with +cards; unless you have already drawn your deck, then you are almost always going to want those cards right away. Or at the very least; you will always want them at the start of your next turn. So I think +card tokens will end up acting more like Wharf or Haunted Woods.