I'm willing to cut the Covenant a little slack where Scarab's are concerned. I played the Halo RTS and let me tell you a Scarab Walkers tear their way through UNSC vehicles like tissue paper. Considering that 40k is basically the Dark Ages with Super Science technology, I'm willing to say the Scarab could pose a serious threat even to most of standard Imperial vehicles.Really the only vehicle of note that the Covenant have is the Scarab, but let's face it. It's only got one serious weapon, and against a moving target it's not that effective. Compare that to our favorite mac-mansion sized tank and it's 11 barrels of hell. Using a Warhound Titan against a scarab would be serious overkill, i'd say a Leman Russ or possibly even a Sentinel with a Lascannon and a bit of patience is all that's necessary to take those things down.
Problem is, it's the only serious threat I can think of, and Scarabs are pretty rare. Focus fire from a trio of Leman Russ Battle Tanks would be more than enough to take it down I'm sure (that much faster if they've got Basilisk support, which they will). It's maybe a match for a lone Warhound Titan, but seeing as how they're employed in scouting pairs and supported by the larger titans . . . it doesn't look good.
The Covenant Ghosts and Banshees might do a lot to harass an IG regiment's flanks, but seeing as how they can be gunned down with concentrated small arms fire . . . Well, they're basically open topped vehicles with an armor value of 7 or 8 (lower than even the lightest vehicles in 40k).