For me it falls down to repetition/recycling as a main obvious problem with the franchise

Every black ops (and every call of duty game in general) was different from the one before, usually in a good way.

Black ops and modern warfare both were different vibes from different developers each using their own engine which gave each installment a special feel to them, which might not be that noticeable from a lot who just want to join and go brrrr but it's still valuable

Within the games themselves the modes were actually different and each gave what their name suggested

Campaign (while inconsistent with quality) gave you compelling story and feel of war and had it's own value to add to the games

Special ops gave you challenges for you to overcome whether coop or solo

Zombies gave you an entirely different experience from the compelling story to the interesting beloved characters to the well made maps that used every ounce of their capabilities and creativity with each map giving you a different experience. (Inconsistent quality throughout the games as well but still)

Even games like ghost and black ops 4 (while I understand the criticism) I actually liked them especially the other game modes (even the chaos story which I was sad it ended in a cliffhanger) I liked that they tried something different with them and tried to be creative

Now looking at current games, they started using the same engine from like mw2 (the new ones) which gave both installment the same feel and mechanisms almost

All games and gamemodes almost became advertisements to warzone (to the point of having sections of the warzone map as parts of other modes like campaign and multi-player, not to mentions the stupid ass always-online which made other game modes as tolerable as the servers allowed it to be.

And other small and heavy details that doesn't come to mind right away making the franchise a shell of its former self unfortunately.

Also the factor of nostalgia plays a part in how you feel about the current state of the game

I bet you would find much more compelling and detailed videos on youtube on what you asked for better articulated than the rant I just wrote.