I swear it wasn't this hard 15 years ago 
I would chalk it up to being 
er but I can still jump into a random game of Gears of War and do okay, so who knows. I guess the skill ceiling in the old Halos is just much higher due to the battle-hardened diehard audience left playing them?
I struggled a lot when I first picked it up but if you commit it'll all start coming back to you. Unless you've decided to play Halo 1, then you don't stand a chance against their godly tech and game knowledge.
I wonder if my issues are made worse by the fact that I'm on mouse and keyboard. I feel like my headshot weapons are not hitting anything and the giant reticles feel kind of awful on mouse and keyboard paired with no aim assist. I try to put the giant crosshairs over my enemies' heads and it feels like my shots are landing all over inside the circle. Meanwhile I'm getting perfectly fourshotted constantly
The Halo 3 Battle Rifle feels especially terrible on PC in 2023 where it felt pretty snappy way back when. To be fair, I've never been an accurate shot and the reason I do better at Gears might be because it's mostly about dashing right up to someone's face or back and gibbing them with a hip fired shotgun
I've personally had the best luck with controller, aim assist is cracked in MCC.
I don't think I can acclimate back to controller. It's a shame the aim assist is what it is since mouse and keyboard feel so good in Campaign and Firefight.
It's also weird to me that of the two Xbox franchises I used to play, it's Gears of War that's easier to play on mouse and keyboard when one is a standard FPS and the other a third person shooter with quirky movement mechanics revolving around animation cancels