They can stop armies at a choke point, keep zealots and dark templars away, and take down buildings. Overlords are required to spot dark templars, wraiths and lurkers. Terran Marines and medics. With the advent of the medic unit, marines are now very tough customers. Marines can effectively enter a fight stimmed without the health sacrifice.
Effective defence requires micro-management.
Most effective terran armies include a science vessel to wipe out annoying guardians, take out a protoss army's shields and detect dark templars, wraiths, and lurkers. Despite claims or theories to the contrary there are no unbeatable armies . There are no unfair units. Every attack has a counter attack that can beat it. You need to build up experience to learn how to beat any given individual attack.