The IC's are among the scarier opponents for Snoic to space.
Their grab game is not as much of an issue especially for Sonic who excels at maintaining spacing from the opponent. So they should rarely grab you (though if they do it can easily be a stock).
A main issue with the IC's is that they are extremely good at playing defensively.
De-synched blizzards do an excellent job at fending off opponents and make it difficult for you to get to their blind spot.
Their ice shot is pretty good too (though remember you can just jab them away) but their purpose is to bait the opponent and pressure them into close range combat where they will beat out most opponents.
Their weaknesses are very exploitable by Sonic, primarily the ability to separate Nana/Popo. Multihit moves work very well against the IC's in this respect, so using the Usmash, tilts, Fair and then using your Bairs to continously keep them away.
As Infzy said, they aren't helpless, they can teleport the IC back to them but this only occurs for a certain range (with the ^B havig the most range an d being the most risky).
Off stag they have a tough time with opponents because their recovery has so many dangers.
if Sonic has an advantage, I definitely don't see one, however, I don't see any from the IC's either.
if you are going for grabs, do so when they are separated or one of them is dead.
you can pull of an Fthrow while one is around, but its generally not worth it.