Overratedroth.

[Sephiroth]

Actually there is no explanation for it. Sephiroth just wanted to restore JENOVA. And both Sephiroth and JENOVA control reunion by their own will, not the cells itself do it. So they can decide when they want to start the reunion. And it would be idiotic to start the reunion before the story starts because Final Fantasy VII needs a story.

But Hojo wanted JENOVA to show him the reunion as well. That's why he placed the body in the headquarters. He thought the body would be the main part for the reunion - so the remaining cells would go to the place where the body is. But no, the head is the main part. Sephiroth had JENOVA's head and he controlled the cells so he wanted the head to call Cloud and the other clones.

The reunion of Advent Children is another form of the reunion, like I have already mentioned. It is exactly what Sephiroth does when he shapeshifts JENOVA's headless body into one of his own. I don't know why they decided to call it reunion as well (because they haven't explained) - but now it is how it is. Actually this shapeshifting ability is way more powerful than the reunion because Sephiroth can create an exact copy of his own body and so he is reborn with a new body and with his original will which makes him the "old" and original Sephiroth as powerful as ever. He doesn't even need a new complete reunion. However this shapeshifting ability is already explained by Sephiroth, Iphalna and the Ultimania Omega. It's cool Sephiroth can use it with JENOVA cells which aren't even his own. So it is more powerful. Would Sephiroth still be able to control Cloud he could even transform his body into a new Sephiroth body for his own purposes.

However, Sephiroth doesn't have all these power and abilities without reason. He is made to be such a cool and powerful antagonist. So he is definitely not overrated. But what should I say. I hate this ridiculus anti Final Fantasy VII hype, like the anti Final Fantasy VII "hypers" hate FF VII.