I've played Sweden, Prussia, Poland and Austria, and each time have found myself in a war with the Russians just about to the bitter end. Even after losing all but the crappiest of their provinces they have a marked tendency to either not want peace at all, or declare war again after a few turns and get hammered, again. The only exception was when I got so annoyed I took all but 1 of their provinces and made them a protectorate. Having given away most of their land to my other protectorates or in exchanges for Caribbean islands, they continued to war on my protectorates, but after 10 years they hadn't declared war on me again, perhaps a record.

I wasn't interested in taking their land (except as Sweden I think, a victory condition), but found myself at war with them each time. And since I wasn't going to give them all my land, all my money and technology we just stayed at war.

I have also found that, for example, Denmark always likes DoWs against Hannover (inevitably my protectorate) after about 1730, when I am 20+ provinces. They don't last long, and only once have them moved troops into Hannover.

The ridiculous DoWs are not only a major annoyance, they markedly decrease the gaming experience. Why would a tiny 1 province nation decide to declare war on the largest most powerful country in the world which is right next door? And then decide to just sit around within its borders waiting for the stacks to arrive and obliterate it (sadly there is no massacre population option any more).

The number of times that has happened is countless.

Then there are the completely bizarre and seemingly random ones... like UP against my protectorate Courland (Courland didn't even have a port!).

And, as has been mentioned, if you get peace with a faction, all your allies and protectorates remain at war with it while you have moved on and are trading with them again. So they keep blockading your trade with them.

So much promised, which fell far short of what was delivered. It could have been so much better.