Power is important, but so are ninjas. You need to balance that, because if you go all-in for power, you will lose to weaker people but with top quality teams, as well as if you go all in for ninjas, you will lose to middle-tier team with more power. My suggestion is look at the best teams for the main you want to play in the future and start piecing those ninjas (if you have any doubt, you can ask here for advice too). Ideally, you would want to spend your coupons in either lucky board or mission general mobilization events, where you get a lot of power items from board/boxes, and you can redeem some fragments of those desired ninjas (but only them, not some random ones) with points. That's the best strategy if you want to stay competitive.