Wow, I completely forgot I even made this thread. In an attempt to revive it, here are all the match-ups that I think the IC's are at a disadvantage against.
Falco:
His lasers alone completely dismantles your approach. They go over ice blocks, through blizzards, stops squall hammer, and disrupts any desynch attempt. SH phantasm is also a real pain to deal with since the IC's have no move to counter/punish it. Dair spike is devastating and will hit you straight out of your squall hammer if you try to recover with it, and it can even hit you out of the startup of belay.
Marth:
He out ranges and out prioritizes you on the ground and air. Most of his attacks are double or more of your throw range allowing him to easily space you, and he has some good options to stop your approach. SH Fair will go over ice blocks and hit straight through your blizzards and squall hammers. He also has a good arsenal of edgeguarding tactics to disrupt the IC's recovery.
Meta Knight:
This is probably the IC's hardest match-up thus far. He beats you out in the air and ground, his speed and size make him hard to keep out, has good aerial mobility allowing him to space away from grabs, has ridiculous edgeguarding abilities to easily gimp their recovery, and the IC's have no reliable way to stop tornado spamming. Everything about this match-up is simply hard.
Pikachu:
Spot dodge into Dsmash can wreck the IC's close range game. Dsmash is hard to DI out of and easily separates the IC's. Thunder is also a good move to separate the IC's, and can be spammed in case they get grabbed in which the thunder will disrupt the combo.
Pit:
Just like Falco, Pit's arrows will go over ice blocks, through blizzards, stop squall hammers, and disrupt any desynch attempt. This just completely disrupts your approach, and his good edgeguarding allows for early gimps.
R.O.B.:
Just like Pikachu, spot dodge into Dsmash will do wonders at punishing the IC's up close game and separate them. And just like most of the characters the IC's do bad against, he has good edgeguarding allowing for some gimp kills.
Snake:
He completely destroys the IC's on the ground with his tilts and jabs, and his Utilt makes it risky for the IC's to go into the air. He can also hold a nade to prevent the IC's from grabbing him because it will explode midway through the combo.
So far these are the only match-ups I see the IC's having a notable disadvantage in, but there may be more match-ups where they're at a slight disadvantage. Despite all the things the IC's have going for them, they'll still end up mid tier at best because of the sheer number of bad match-ups they have, especially against the higher tiered characters.