Alright, I just spent a lot of time messing around with Ganon's sideB. Here's what I think about it:
It always guarantees a follow up. However, this is no different from his Dthrow. The main difference is you can chain grab any character with it, on reaction. However, there are a few differences than this and Sheik's Dthrow. First is the initial dash. This means you're committing to it well in advance, and your opponent can take advantage of this. On the other hand, you can also start the chain grab in the air, which is a pretty big boost. If the opponent techs the sideB, you have a guaranteed follow up sideB. However, if they land, things become a bit trickier. They can roll, get up, or attack. Rolling behind you or getting up result in a regrab. If you position yourself outside of getup attack range, then you're also guaranteed a regrab. The problem with this is they can just roll away from you, and you can't regrab them (I don't think you can reach them in time with Wizard's Foot, either). If you shield, then it's an easy regrab after a getup attack, and you can SH SideB anything else. However, unless you're insanely fast, they have enough time to throw out something quick, so could stop the SH sideB. Otherwise, you can keep grabbing. Also, if there's a platform above you, they get placed on it. This is a pretty nice situation or Ganondorf (his opponent's above him AND in knockdown), and guarantees a follow up, but it doesn't guarantee a chain grab.
My conclusion is, the move is extremely good, but given that it's on Ganondorf, it means it won't make him broken. You also have to find an opportunity to hit with the move, and given it's speed, it's more difficult than it sounds. It will make him pretty good, though. Not to mention, all those guys who had a big advantage on FD aren't going to be taking you there anymore.