Ammo for weapons is plentiful in CE most of the time, if you manage to run out of ammo for UNSC weapons, that means you are fighting the Covenant and the Covenant has a weak capacity to engage at medium and long-range, only NPCs with a Wraith or a Fuel Rod Gun have the attack range to engage from far and they aren't reliable.
If you can exploit that weakness it's good. The A.I. isn't very good at leading it's shots so you can even beat an Elite with a Plasma Rifle.
Halo 5 is by far the easiest legendary campaign. Projectiles aren't that fast and the ai is nowhere near aggressive as H2, if anything H5's ai is defensive, it will virtually never chase you down. You also have a constant 3 allies at all times that you can order to focus fire with you and carry around powerweapons. Oh yeah, they'll also revive you if you somehow die giving you infinite get out of jail free cards. The only time you will ever actually die is when your squad has a 10IQ moment and all lines up to get killed by whatever downed you. Everything that makes the H5 campaign unique is a literal crutch
It cannot be with Combat Evolved and (mostly) Halo 3 behind, the A.I. was also always defensive in all games, being usually bound to an area and even rarely moving to a different area. Reach has a noticeable exception if you ever activated the switch to get a Pelican or Phantom, the Banshees can chase you far from the mission area. This is not an effect of the switch.
Those aren't melee or berserking enemies, the exception which makes NPCs leave and chase a target but annoying and dangerous enemies (if caught in a bad situation) .
In Halo 2 they like to shoot at the last known position, mostly concerning if the attack is explosive or instant-kill. Aggressive but more annoying than dangerous in pratice.
Unique like the infamous battles again the Warden ? In Halo 2 it was more fair, being made for a single character in mind, this is what up the difficulty above Halo 2, you can order to focus fire and carry power weapons but the levels being designed (enemy placement and all) with coop in mind, balance that advantage quite well. I won't count the Warden fights because that's unfair for a solo player.
The management of allies NPCs to fight better is a headhache, even in the Bungie-era you could get some serious -3000IQ moment on vehicles where you need a gunner. Halo 5 is no exception but is not limited to vehicles anymore.