if roaches do small damage, just kill the offensive enemy and rape the roaches later
None.
roaches aren't a problem. they are EASILY beat down by focus-fire(which any mediocre player should be DOING ANYWAYS) and are BIG(not ultra-sized, but they're quite big alongside lings). the only race that will have issues are the protoss with their zeals(on a very small scale), they don't have range or the ling's small size.
also, tanks one-hit KO roaches(50+50 armor bonus > roaches' 90 HP).
None.