The Imperium has over 10,000 years of experience in fighting every known alien threat....yeah. They're good at it. I mean, we can look at their most basic weapons. The Lasgun. It is a laser rifle that, with a single shot, can punch through armor, decimate modern armors, leave head-sized holes in concrete, and leave targets a particularly bloody and cauterized mess, all in one shot. Doing about the equivalent of a modern-day .50 cal rifle with almost no recoil, and as a medium-long range semi-automatic assault rifle.
And that's among the Imperium's weakest weapons, hell, the weakest gun in the setting, arguably, having earned the moniker "flashlight."
So what do the Covenant have? Essentially extremely scaled-down versions of Laspistols, with the only troops able to do anything of importance against a squad of Guardsmen being deployed as special forces or squad leaders. Effectively, we have 10 grunts, 1 elite, and maybe 2 or 3 jackals against a squad of Imperial Guardsmen armed with the equivalent of rapid-fire UNSC Sniper Rifles. Now whether or not Flak Armor is any better than UNSC Trooper armor is open to argument.
Now consider the fact that Guardsmen are considered expendable, and it is the tactic of Guard commanders to drown their opponents in bodies. Also consider the fact that the Guard has access to the best tanks in the setting.
Essentially, what the Covenant are up against are a rampaging tidal-wave of humans who are, individually, actually more potent than their opponents, and ABSOLUTELY HATE THE COVENANT'S GUTS backed up by vehicles and tanks the Covenant has no defense against. Bolters, AKA Automatic Rocket Launchers, are standard-issue mountings for the Omnissiah's vehicles. And that's only a secondary/sponson mounting.
Now this is for the ground war only, but I seriously doubt it'd be at all different in space, with ships that can fire shells that 40k novels have literally described as "apartment-building-sized."
TL;DR, yeah. TOTAL blowout. Honestly I think all the Imperium needs to wipe out the Covvies are a Battlefleet and a few Guard regiments. If the Astartes get involved, let's just say Khorne will be pleased.