Tactic cards appear in the game during your turn as well as the opponent turn.
You can play them but my problem is in particular during the early stage you simply do not have enough Resource tokens to pay for the cost of Tactic cards.
For example lets assume it is your second turn of the game and during your turn you get 3 Resource cards as per the Capital board, you will have to spend 2 or all of them to put down
a Unit card now you have no resource tokens left when the opponent starts their turn to
use your action cards during his steps of play.
I played against a stubborn chaos player and she just would not let me put down cards in my Kingdom section. (I was trying to increase my resource tokens for future turns and she kept going after them and destroying them and I was constantly faced with bring in only 3 resource tokens which most of the times there was no enough left for me to spend it on the Tactic cards during her turn.
So unless i got the game strategy wrong, Tactic cards are hard to use.