All_Under_Heaven is right. Halcyon class cruisers are slow, unwieldy, and basically a joke in the UNSC. No self-respecting captain would ever have one. They have 1 MAC gun that is seriously underpowered, and only a few Archer missile pods. A standard Halcyon-class cruiser's only good pint is that it is the toughest ship in the UNSC arsenal. A Halcyon class has almost no empty space inside of it and has even more Titanium-A in it than a destroyer. It has an immense system of honeycombs that provide it with unmatched structural rigidity and strength and hydraulic braces that can be deployed in an emergency to fill in for missing or crumples honeycombs. I believe a Halcyon class can absorb more damage than a Marathon-class cruiser.
The Pillar of Autumn is an ONI covert-ops mod job. It has a specially refitted MAC gun, additional capacitors for it, magnetic field recyclers to recapture lost energy from the magnetic fields when it is fired, the fleet's most advanced reactor providing it with unmatched speed, as well as its ability to be pushed to 300% of capacity with only increased wear-and-tear to be concerned about due to its prototype cooling system that uses a laser-induced optical slurry of ions cooled to near-absolute zero to remove waste heat so runs off the reactor itself so that the higher the reactor goes, the more coolant it produces. Oh, and I forgot about the entire starboard docking bay replaced with Archer missiles, the massive amount of point-defense cannons, having two AI's on board, and the nuke-equipped Longsword Keyes uses to destroy the Covenant flagship at Reach. The Pillar of Autumn is the furthest thing from a joke as it packs the firepower of a Marathon-class, the speed of a frigate and the toughness of a Halcyon-class all in a one-of a kind package. In my opinion, a PoA style Halcyon should be the best capital ship the UNSC has, able to go toe-to-toe with a Covenant heavy cruiser and win.