Biggest problems of Halo 4 looking back for me, loadouts, global ordnance/personal ordnance, sprint, flinch, plus all the small things we don't remember (instant respawn, no working file share at launch, no x on death, no non-Infinity playlists for a while).
I think just the fact they let people customize everything about their weapons lead to such unbalanced gameplay. People with ammo perk had 6 incineration cannon shots, people with stealth perk and camo would get sniper, die, and then become almost silent and invisible on maps, people had anti-vehicle loadouts (plasma pistol, plasma grenades). They could die by a warthog gunner, instant respawn just 20 feet down the map with their plasma pistol and plasma grenades, take out warthog etc.
Personal ordnance lead to entire matches playing like action sack. 3-4 players would have sniper weapons in play at all times, rockets, splasers, incineration cannons, binary rifles. Global ordnance was so poorly implemented, you didn't even know what was where half the time. I distinctly remember losing a match because blue base on Exile got a fuel rod gun right near their base, took out our driving warthog, whereas our side spawned a needler.
Sprint negatively impacts Halo gameplay. We've had these posts before, dozens of them. They impact maps, they impact gunplay, they impact map control, spawns, promote cat and mouse gameplay, maps were ridiculously large because they had to account for sprint. I'm sure you noticed it. Became just like CoD, instant respawn, sprint, die, change loadout, repeat.