Well, Flash's rogues often fight as a team which is what makes them a threat to Flash. A team of Mirror Master, Captain Boomerang, Heatwave, Weather Wizard and Captain Cold is no joke.
Anyway, individually; Captain Cold's gun produces a cold field which slows down Flash. He also freezes the ground around him which makes it difficult for Flash.
I find it more difficult to defend Captain Boomerang. At times he has had some super-speed abilities. For the most part he is about trick boomerangs. Gas boomerangs, Electrified boomerangs, I believe gravity ones too.