Stop playing games on "easy", lol. But even I found the campaign to be a bit short, almost similair duration to gears of war.in the modern world of awful shitty games like medal of honors 2 hour single player and shite multiplayer, 4 hours of single player and 4 extra maps to a working multiplayer is 2 full games worth of content, you should consider yourself blessed.
When you compare the books (so the actual Spartans and not the player controlled version in the games), the Spartans seem to be more acurate in shooting and have more prowess in combat than the average 'nilla marine. The iniative should be high because they seem to have a much lower reflex time than the Space marines.
you should always use the books (maybe Halo:wars aswell)as the base for the UNSC units because in the books you have the halo universe and characters as bungie wanted them to be, not controlled by players of different skill and playing style.