Treyarch games have notoriously always had worse bullet detection and connections. This was the most glaring when we went from the very clean MW2 to the first Black Ops game. I could only play core locale only in that game. Thankfully HC mode works great in 3arc titles but core is always iffy. Now that there is also evidence that people are being artificially impacted mid-game, it just adds to the usual problems associated with this studio. If you're a half decent and experienced player, HC mode is also ideal for easy kills with minimal effort - you just have to play sensibly and take into account how quick it is to die otherwise you can literally pop someone in one tap.
Having said that though, I never felt the need to resort to this mode in Cold War because the bullet detection wasn't too bad - the worst aspect of the combat I found was the animations and trying to track people. Sometimes they'd skip frames and what you see isn't what they are doing. Both studios have always had their own respective engine builds so I imagine this is the big factor.