This is all on very hard/very hard so it might not hold for lower settings. These are my main diplomacy observations.

1. So far as Carthage, only Germania would ally with me. They broke the alliance on the following turn. I'm at war with all the Romans of course. Other factions that are at war with Rome and not allied to any of my enemies, refuse alliances. This does not make sense to me. There is no harm in accepting.
2. Every faction I share a border with will initially will grant trade, then attack me just as soon as they can get an army in contact with anything of mine. WTF? This is stupid for Numidia, a traditional ally. It costs them too much financially to start such a war.
3. I can get ceasefires after kicking the dog snot out of a faction--improved from MTW.
4. I have had the most territories since early on in the campaign. Everyone attacks me if they share a border.
5. When not at war with me, all factions I have asked have accepted a trade agreement. This is a positive!

Mixed bag so far.