I experienced numerous bugs and frustrations related to diplomacy. 

I went for a single massive city (EL cultist style). I had 3 countries around me, and 2 of them became vassals of the third. Since there are very few diplomatic options, the only thing I could do was declare war on all countries, and since there seems to be no way to artificially create war desire (demand money, insult, etc), I was forced to declare a surprise war (declare war greyed out). After this, I experienced a lot of bugs and frustrations. 

So I was surrounded by a liege country and its 2 vassals. Upon declaring war on the liege, the other 2 changed to war as well, but I was unable to enter the territory of the vassals, with the message that I needed to declare war. Saving and loading solved this problem, and I was able to invade their territory.

Upon conquering all of the lands around me owned by all 3 nations, I was unable to gain much ground at all in the war resolution, despite effectively removing them from the map since the highest it looks like you can demand is 180 points. Since a city is 80 and a territory is 40-ish, this necessitates declaring multiple wars over time. While this could be ok if there are ways to generate war desire, the lack of this and the fact that any war resolution sets war desire to 0 meant that there was no way to gain enough war desire to conquer multiple cities effectively, even over multiple wars. Also the way that the territories appear in the war resolution window (simply having the name of the city) makes it unclear that it's just a territory, and in the event that a city has multiple territories, you're not sure what ones you are getting. 

Also, I regularly encountered a bug where despite being at war with one of the nations, their diplomatic state suddenly changed to at peace, despite me currently occupying their settlements. It seems like they suddenly wanted to trade luxuries, and this somehow forced the peace state. This created a limbo for those cities, since I can't do anything with them while they're occupied, but I can't demand them either since we're at peace. Even when I declared war again, they didn't show up for quite a while, and I'm not sure why they suddenly appeared. 

Last and a sidetrack, but the influence cost to absorb even a very small city is insane. to absorb a 3 population one territory city into my capital was listed at 19,000 influence. I believe that the most recent neutral territory that I added to my capital was about 160 influence. It should be expensive, but that's really high. Something more like 2000 to 4000 seems reasonable.