I was playing a game as the UNSC, against the Covenant on a huge map. it was a 1v1, but I used the huge map to simulate Humanity expanding into a vast number of colonies and promptly being squashed by the covenant.
After being smacked around for a while, I found that using a fleet of 40 Autumns with assorted support ships (charons + cradle mostly) could steamroll just about anything the cov could throw at me. I only had half of the fleet size upgrades, but was easily able to beat any number of covenant fleets with minimal losses to myself. In addition, the spartan boarding party ability allowed me to recoup most of my frigate losses by stealing enemy ships.
TL;DR Is the autumn amazing or does the AI not build/research very intelligently?
Side note: is there an easier way to upgrade that many cap ships other than clicking on each and every one of them and selecting the same upgrades?