While technically they are built up of several races, from the Grunts all the way to the Brutes, the Covenant is a fearsome group indeed. With an army of front line fighters who are willing to blow themselves up for their cause and higher ranking generals who are know for their tactics as well as their brute strength, they have most their angles covered. It is safe to assume that they will stop at nothing to stomp out the Human race.
With the multitude of races gathered in their ranks, their technology is more advanced than we are capable of to match. Mixed tactics, greater technology, and religious zealots behind their purpose for fighting, the Covenant is one race to prepare for.