The two recent games I've played as test games with the 1.8 patch are probably the most frustrating ones I've ever played.
First one was with Sweden. Got independent, conquered the Baltic coastline from Novogorod and all of Livonian Order. Converted to Protestantism and everything was fine.
Until the league wars.
I had been at war with Denmark when the leagues activated. Since I couldn't join one, I sat back and footstomped both the Danish and the russian army whilst the Leagues began to grow. A year had passed since the leagues were created when I finally peaced out of my great nordic war victorious and thought that I perhaps should go ahead and join the Protestant League. But when I mouse over the button to do so: "England has declared war on Austria, they cite 'Religious League War' as their casus belli'". Yikes I thought, didn't have time to join it. Perhaps I'll just wait and join the league when the war finishes (I was completely unaware at the time about how everything worked with the leagues). I then went to diplomatic map mode to check how red Europe was for England.
And my god was it red.
Austria had a very OP Poland and Lithuania, all of southern Germany and Portugal with them from the start.
I thought to myself then, maybe it isn't that bad, they might be able to pull it throught.
I have never been so wrong since I'd started playing EU4.
It took the Catholic League
3 months to kick the protestant leagues' ass and the only one left was England. Meanwhile, super blob Hungary, France and Castille had join the catholic league (because anyone in the Catholic League can call in their allies which unbalances everything instantly) and smashed everything England could possibly put up against them. Somehow England was able to occupy Île-de-France for half a year without it ever being sieged by anyone. I then ragequit because I had missed joining the league and also for Russia and Lithuania simultaneously DoW on me with 6 6 generals and I got stacked wiped with my very best 4 3 general.
So I thought maybe I should play someone in Germany to be at the front line for the religious conflict?
That's when I decided to play Brunswick and try to form Hannover. That wasn't too much of a problem, conversion wasn't too difficult, had Denmark supporting me at the start until when I backstabbed them as Sweden declared Independence, switched ally to France as I usually do as Brunswick.
Then Burgundy died and I hadn't noticed that Austria wasn't emperor for some reason unknown because the Palatinate got it all. So I thought this should be easy then with the Leagues and everything
But oh boy, things turned bad faster than lightning.
Austria was back as Emperor around 1510, which is starting to get close to the possibility of the leagues. I didn't really mind it as I still had France as an ally, but of course, the game hates the human player and France switched me out for Austria. Fack!
So in 1545 I decided to cheat. Since Austria was still emperor and France seemed to lick their boots, I tag switched to Austria and set France as a rival. This however didn't help much, because a few month later, The Palatinate was back in charge. Now I saw an opportunity to win the religious war.
And the leagues triggered.
Again I was at war with Denmark and couldn't join it at the start. However, this time I quickly peaced out and joined the Protestant League quickly. When we had a clear advantage over the emperor who only had Trier and a couple of one province minors with him, I decided I should attempt to start the war.
And the game of course kills my ruler, leaving me with -1 stability which quickly turned to -2 through an event and a regency for 5 years. Darn.
This was enough time for the Catholic League to gain powers such as Lithuania, Sweden (dafuq mate?), Castile and Portugal. England joined the Protestants with Novgorod, Denmark, all the minors in the Netherlands, and most of Northen Germany.
Now when the Catholics had been let to grow overpowered, England decided it was time to start it all.
The protestants were winning at first. Since Castile couldn't get through France, the catholic HRE forced were smashed at the start giving my side a nice 30%. And out of the blue came Austria with two 5 6 generals with murder stacks and moved the warscore down to 20%, but we were still doing pretty well.
That is when France also joined the Catholics.
Half a year passed and I was stack-wiped twice, half of my glorious Hannover was occupied and France-Austria-Castille-Portugal were turning Europe into a Protestant blood bath. -70% warscore in less than a year... ragequit.
It may be because I'm inexperienced with the new DLC and everything, or the Super Power AI shouldn't join wars from a OPM call to arms were the battlefield is turned to 100% in favor of the enemies' side when the enemy really doesn't need any help at all and probably could handle the situation for at least another 10 years

.