The reason why A Show of Power is bad is because it gives you very little advantage. In theory, it looks amazing. -1 from Hand, -2 Chakra, and a Damage to their Ninja. However when you actually play it, it's lackluster.
The -1 from Hand is only good when they have 1 or 2 cards in Hand. It's especially not relevant when mass draw such as The Warhawk is around.
The -2 Chakra is only good when they have about 3 or less Chakra and you have explosive Jutsu, which will almost never happen because you're playing Water. Also, because of cards like The Warhawk, Chakra will just be built up on the next turn.
The 1 Damage is, in my opinion, the worst of the three effects it grants. Your opponent will most likely give the Damage to a Ninja they no longer have need for and plan on turning into a chump blocker, or in some cases, give it to a Ninja such as Naruto or Obito, both of whom are popular cards in format, making it stronger.
The worst part of the card is the fact that you go -2 in Hand. This is detrimental in a deck where you have no way of refilling your Hand. Draw power is scarce and your best draw card is a Counter Mission that relies on your opponent. Your next best draw card requires you to have less cards in hand than your opponent to simply break even.
I'd run Moment of Weakness over A Show of Power any day. Why? Because Moment of Weakness gives me an option of the target, to a degree — I wouldn't use it if they had something I didn't want to waste it on. What's better is that it gives you such an advantage for so little cost. On Turn 3, if you went first, they are going to be at 2 Ninja, 1 if you dropped Utakata that turn, to your 4. If you went second, they will be at 3, 2 if yo dropped Utakata, to your 4.
Now, this is all in theory and requires you to not miss a drop, but the chances of this happening is a lot better than the chances of your opponent having a few cards in hand, a few Chakra, and very bad board presence. Perhaps the best part of Moment of Weakness is the fact that it returns to Hand if you hit a 0 drop. It practically is a -1 from Hand to snipe a Ninja. The next turn, you play it and hit a 1 drop. -3 from Hand to kill 2 Ninja.
You can play A Show of Power, it just won't do much. Water shouldn't be focused on gaining Chakra or playing big Jutsu this format; there's a much better way to pilot the deck.