The diplomacy is not fully explored in 1.5 yet and even one whole campaign is not going to give you the feel for what it "usually does".

I have had countries with death wishes and I had a nation begging me to become a protectorate if I would just end the war.

The agent spawning was more to my liking before, the units and upkeep were fine before, the economy over all was working fine after the mess in 1.2.

There are a lot of things I liked better in the game in earlier versions.

But as to the diplomacy I don’t have enough playing time to call it broken.