We don't know whether there's a cooldown for Uranus and Pluton, meaning how repeatedly you can use it. As for Poseidon, not all Sea Kings are created equal. Some Sea Kings are humongous. Here's the size of the Noah:
A lot of the Sea Kings are of that size or above:
Commanding an army of them is quite sick.
Then again, we don't know about the cool down of Uranus and Pluton. Pluton too has to travel, Uranus probably too.
You of course can still argue that Poseidon is the weakest of the ancient weapons, but they are close to each other.