spending coupons on power will give you more or less the same chance if you get better ninjas with higher growth rates, a f2p 60K+ bp with basic/coin-shop ninjas can't beat my fire teams consisting of Hanzo danzo and sage naruto etc for example, however some ninja that are good as well as really cheap like Sailor sakura and 1010 gnw can be effective with the ''only spend coupons on power'' strategy.