There are also "further tweaks and improvements" to diplomacy, missiles, melee units "and much more." To take full advantage of all these patch changes it's best to restart campaigns.
Not everyone will though and that's fine, with Creative asking for any feedback if problems arrive with old campaign saves. They also caution about mod incompatibility.
"This update includes major AI improvements related to the constructions of new buildings, food supply management, creation of armies, technology researches and tactics used during battles, making the AI more organised and challenging. Further tweaks and improvements have been made to the diplomacy, missiles, melee units and much more."
"To reap the full benefits of every patch, it is recommended to start a new campaign. However, please try your old campaign and see if you benefit from anything advantageous. If not, start up a new campaign and compare the differences/similarities. Your answer should lie there."
Total War: Rome II's wiki lists the- it's rather long. Work has begun on Patch 5.