I think there are trade-offs on either side that keep the enemies pretty much equally bullshit. Overall, anyway. Halo 4's enemy sandbox is a little less diverse than 2's, as much as it pains me to say that, but the sheer frequency by which they throw Promethean units at you in the tail end of the campaign just make it ridiculous. Halo 2's Legendary AI is definitely the more accurate and aggressive of the two, like you said, but Halo 4's enemy units get to a point in Legendary where there are more enemies that can instakill you than ones that can't in any given encounter, and on top of that they substitute aggressiveness for outright persistence. Prometheans being able to revive, birth out more Watchers, make themselves immune to headshots for a couple seconds when they aggro, teleporting away if they're about to die, etc. just makes it a nightmare unless you can accurately manage all the Promethean units on the field. Prometheans also have their better weapons being shared across all classes rather than distinct power weapon distribution like Fuel Rods = Grunts while Energy Sword = Elites, etc. Crawlers can wield Binary Rifles with next to no visual indicators on rank. With draw distance obscuring the (not really) obvious tells with Promethean hierarchy like the electrical smoke stuff on higher classes, it's next to impossible to figure out if a Promethean's wielding a Light Rifle, Binary Rifle, Incineration Cannon, etc. until you actually get shot at.
While most Halo 2 Legendary encounters I struggle with typically involve a company of Jackal Snipers or entire groups of enemies making combat overwhelming, the thing I struggled with most on Halo 4 Legendary was a one-on-one encounter. Trying to get through the final portal in the "portal hub" room that eventually gets defended by Knight Battlewagons is just ridiculous. They have a Battlewagon wielding an Incineration Cannon sitting right on top of the final portal, so it's pretty much impossible to cheese the encounter by avoiding it or running past it. You have to kill it, the Battlewagon has the upper ground, and cover is negligible - you can't really draw out the encounter to your advantage to slowly chip away at its health, because ending up just about anywhere within the blast radius of the Incineration Cannon will get you immolated.