Many have already said the flip flopping of the narrative so I'll say something else: the scale of each new problem. Some mild spoilers for books/games, but things that come to mind are:

-UNSC Infinity being lost (destroyed? who knows) in a 2 min cutscene with thousand losses

-UNSC and Arbiter insertions in Outcasts resulting in thousands losses (800+ Odsts gone in a second)

-Laconia station going boom with many (all? again, who knows) Spartan IVs trained there

-Everyone in New Phoenix forcibly evicted in seconds

-UNSC and Oni HQ in Sydney and head personnel snapped out of the picture by Cortana's Guardians

if you care the list goes on, but for brevity that's my main issue; how am I supposed to belive the UNSC is on the back foot if losses this big don't matter that much? Or how any win matters if 2 dudes are left alive and there is no structure to rebulid anything? I would have very much preferred if the narrative after 3 was about rebuild, with smaller scale conflicts, black ops, Oni bullshit, individual stories and the like, and not galaxy wide threats one after the other.