Whoa, there's no way that Robin benefits the least in customs meta out of anyone other than Jigglypuff. Yoshi barely gets better (taking Lick is suicide; you need your top-tier command grab), Diddy doesn't even notice the difference between default and battering banana peel, Meta Knight prefers his default set almost always... there are several handfuls of characters who prefer non-customs to customs. And then there are other cases: Sonic mains will adamantly insist that Hammer Spin Dash isn't worth taking, and the Lucario forums similarly don't like his customs too much from what I've read--you can take these opinions with a grain of salt as those character mains are pretty accustomed to their non-custom characters, but in general there's a grain of truth there (though I think Hammer Spin Dash in particular is underrated because of the hop, but more on that another time).
Robin's Speed Thunder specifically improves her worst matchups, like Sheik. I know @
Nairo at least thinks that Sheik is Robin's worst matchup, and Speed Thunder massages the pain a little. Speed Thunder is a good interrupt, and also a good way to pressure a Sheik who is just spacing carefully and pressuring (needles, f-air).
Thunder+, Fire Wall, and Distant Nosferatu (which conveniently has the same range as Arcfire, for a great mixup) are all great customs too in certain matchups, though of these Thunder+ is the most notable.
Thoron+ basically becomes her new gameplan; it isn't unblockable, but it hurts a
lot, and also KOs. It basically forces approaches, which regular Thoron also does but not to nearly the same extent. You can't just let Robin charge up a 31% damage stage-length KO move. You need to go in against that. So Thoron+ doesn't do anything for matchups like Sonic, because Sonic already wants to approach you (and you want Speed Thunder for Sonic anyway), but I know that as Pikachu when I'm tjolt camping a Robin, I'm a lot more terrified of Thoron+ than Thoron, and I feel the need to go in. I would also totally take Thoron+ against Meta Knight, for example, because Meta Knight likes to hang back and wait for you to mess up.
Since it's only one charge, too, Robin gets the book immediately after using it, which is amazing both if you whiff and if you hit. If you hit a shield, great, now you have another immediate shield pressure tool. If you hit your opponent, great! You now have a KO tool. If your opponent is dead, also great, you have a neat way to start pressure on the next stock. The book is a great followup to Thoron+.