Balancing characters with the addition of effects is very interesting. But in general I agree with Worm38. I think the game could use a slower progression to introduce each character more gradually. I can imagine a series of levels where you only have to deal with basic characters + one character with effects, so you can get used to them. In addition I think some of the effects were pretty complex, and some levels had too many characters...
I think some useful mechanics could be displaying the total weight of remaining characters (the ones you haven't placed), and an undo button.
But anyway! I'm having fun with the game. The art is really cute. I like time mode because you can play it more with intuition.