Stages with too much movement or hazards give the IC's trouble since it'll constantly disrupt them. Here are their bad match-ups:
MK:
This is literally their hardest match-up. Everything about him is almost a direct counter to the IC's general play style. To simply put it, MK has a very easy time separating the IC's and edgeguarding them, which are their most exploitable weaknesses.
Snake:
He simply out ranges and out prioritizes you. His ground game will completely wreck you, and approaching from the air is also bad because of his Utilt. Smart Snake players will constantly hold nades preventing you from CG'ing since they'll explode in the middle of the combo.
Marth:
He also beats you out in range and priority. His ground and air game are both better than yours, and he has superb edgeguarding meaning you'll probably lose at least one IC when falling off the stage.
Pika:
Dsmash will **** you completely. It's hard to DI out of and it will easily separate the IC's. Some Pika players will tend to spam thunder in case you grab them which will probably disrupt you.
R.O.B.:
This match isn't really bad, but spot dodge into Dsmash can be as troublesome as Pika's. Also his edgeguarding can get some easy gimps.
Pit:
Hit arrows will go over your ice blocks, through your blizzards, stop your squall hammers, and disrupt your desynchs. Your approach is just completely totaled.
Those are the only characters who can potentially counter the IC's, they seriously have a lot of bad match-ups.