Everyone here has played Halo right? Remember how you are pretty much invincible every time you hop in a Scorpion? That's why the levels with them are so few and far between, because as long as you're in the tank, the only thing that's even remotely a threat to you is a Scarab.
Now consider that the Scorpion looks like the equivalent of those early WWI rolling canons compared to the Leman Russ, whole fleets of which can be deployed by even modest Imperial Guard Legions.
So, if Master Chief in a Scorpion can plow through entire Covenant battalions, I think it's pretty safe to say that a lone Baneblade could wade through a Covenant army like so much shallow water.
But as you said, it is at that point that the Adeptus Astartes will almost certainly be called in, and with them comes death on swift wings.