I think once per turn is a good rule. You are only able to attack with any one particular unit once per turn, so you should only be able to activate an effect once per turn. Unless of course the effect activation is a result of some other element. (player placing a creature, using a spell.....) Now that effect is based off the unit it is tied to, so if you have 3 of the same units, that effect could be used 3 times. I think that would allow a bit more of a combo. Not to mention we will have cards designed to work with each other. So essentially you will still give the player a sense of control and an integrated form strategy about the game and how they build their decks.