Seriously, they are completely unable of making campaigns that stay challenging throughout, or at least have it not be some snowballfest. If campaigns are difficult, they're always in the beginning.
I have to do the unimaginable and run defense for CA for just a moment. From my personal experience barely any developer and/or game is or stays difficult/challenging throughout its entire runtime. The difficulty curve always ramps up in the begging until it evens out quickly and often drops down for the rest of any game. Which often comes down to developers not understand what difficulty is and equating it with the usual busywork ( stat imbalances, spongey enemies etc). And of course the bigges culprit is AI which has seen no tangible improvements in decades and is often incapable of playing the game at all thus needing to cheat to impede the rapid progess of the player.
CA don't bother trying to improve the situation because why should they ? Im sure all metrics that developers get nowadys point to people playing games on the easier difficulty settings and often even dont finish the game or in the case of TW ( and something like Civ) only playing for a couple of turns until starting a new campaign.
AI cheats it is than to create the artificial difficulty. I bet if the AI would actually understand and know how to play both the strategic and the tactical battles we would have this conversations. But alas...
In Rome and Medieval 2 if you started on one ass end of the map you'd at least know you'd have at least one huge faction to fight against when you got to the other ass end of the map. Hell, in Rome if you play as one of the Romans and don't trigger the civil war early and let it develop naturally you always end up with at least 2 big factions facing you.
Even though it feels more natural and less on the nose it is still an artificial system even if it is a bit more elegant than Realm Divided.
The only way they're able to prevent snowballs from happening is if they put in artificial stopgaps like Realm Divide in Shogun 2 or having public order problems if you as much as sneeze too hard like in Attila.
Speaking of, Realm Divided sucks in its current from, they might have been able to tweak it but never bothered. But apart from the aforementioned big issues (AI) I think CA has been really inconsistent with this kind of stopgap mechanic because most players just want to paint the map for a couple of hours.
And since I am an ardent Attila defender, which really pissed off the whiny baby TW players, I have to say that its systems were great on paper and some even actually good in the game. Having to pay attention to public order, something that can be ignored in all other games ( or even exploited) was nice, I think Attila had the most fitting stopgap systems implemented that also felt natural to the game.
I think CA never really figured out of to implement artificial stopgaps and barriers that felt immersive and facilitated people to engage with the game without feeling cheap or forced.