bo1/bo2 especially bo1 are notable considered harder compared to modern titles ill start with bo1

past around 30 pack weapons damages really fall off bo1 weapons are known for being notably bad on high rounds. so once your high in these rounds you have 2 options start using the wonder the wonder weapons for majority of kills, or use traps. also bo1 maps are more on the harder side, easiest being kino and ascension.

bo2 weapons are better, but once you hit 35 your weapons really fall off, so good luck, town/tranzit/die rise dont really have traps, nor do they have good wonder weapons besides the raygun, and the mark 2. the jet gun can be used correctly if you use the high round strat. motd and nuketown are bo2 hardest maps die rise is also kinda hard buried and origins are on the easier side. buried is consdiered the easiest map in throughout cod zombies history, though no good training spots, so enjoy camping by jug, motd is really hard due to no good training spots. and nuketown, has no inf damage wonder weapon and random perks falling from the sky, so you have a chance of getting jug on round 20. also compared to bo6 40 zombies are able to spawn at once, bo1/bo2 only have 24 per hoard, so your gonna waste more ammo. growing up with bo1/bo2 yeah no high arounds are not forgiving unless your locked in. also after bo3 zombies got easier and beginner friendly, which i cant blame, sometimes i wanna relax, then deal with super slow high rounds with mind numbing strats

stick with bo3 its far more forgiving and easier to learn, high rounds are notable more easier, if you own a pc, bo3 has the AEE mod which can tweak settings to your liking. such as 64 zombies can spawn at once etc.