Calus commands an army yes. But if you read the lore on the raid armor you learn that his military power isn’t what makes him dangerous. It’s his charisma, charm, unparalleled talent for manipulation, an overwhelming instinct for self preservation, and ability to sway the masses. Along with it comes a narcissistic personality and a desire to be the last one standing in all situations with the skills to make it happen.

Not strong enough to survive a saint-14 styled headbutt.

<His Loyalists are about as strong as normal Cabal. The difference between a Loyalist army and the typical Cabal legion is that Calus has his Psions roam free for the most part. Calus himself is a mixed bag. His robots in of themselves are pretty powerless, gaining their abilities from Psion Councilors during the fight. The Councilors also maintain his Nightmare Realm, an illusion created to intimidate and test strangers and/or prisoners. Calus is likely very old. He may be frail or he may still be strong. If his robots are any indication, however, he may not be much more powerful than Ghaul. I mean... he DID use an Ahamkara bone to sway the people anyway. Without it, he may have never taken over.>

He could end the last city and the Traveler if he so desires.

The fact he has his own ascendant plane shows he's up there in terms of power. His robot counterpart was fairly strong in the raid and we go on to see vast rows of the same machines showing he has access for vast resources and not just raw power. Not to mention his collected shadows and forces.