As I pointed out in my previous posts (#3474, #3476) playing with Hungary a successfull game on emperor difficulty is quite insanely hard because of the massive amount of barbarians who mostly destroy the improvements around the cities and don't let you expand.
To solve this problem I recommend lowering the number of 7 strenght lancers by half.
I tend to disagree on this. Hungary on Emperor is truly a difficult challenge, but it is not because of the barbarians - in fact they are the only reason that IMO the game was balanced on emperor and made the game fun. I would go as far as saying that the number of barbarians should be this high on monarch as well.
Dealing with the 7 strength lancers is easy, they spawn by four, that means to defend it, you should have two or three stacks of 4 spearmen with woodland/forest defence upgrade staying ready close to the places where they usually spawn. In the moment they spawn, move the stack to a forest tile close to them, so they are in the lancers attack radius. I can guarantee, that they will always attack the closest units your stack of four spearmen (thats why moving the stack close to them is necessary) and they wont move to your cities or try to pillage your land. I didnt lose any spearman with this kind of defending, this is the best way to deal with them.
As for the later mongol invasion, there is only one tile on east in the Carpathian mountains where they can invade. That tile has 75% bonus defence (hill+woodland), so you first need to build a fort here increasing the defence bonus to 100%. I moved 15 guisarmiers with woodland defence upgrades (although 10 is enough) that held back more than 30 mongol horse units during the few turns they were attacking, then they gave up
I managed to do the first UHV, having the biggest land in europe. Kiev collapsed due to the mongol invasion, I made Poland collapse, and gained the necessary land by conquering Austria. The second UHV - no Ottoman city in Europe - well, I screwed that up. I should have thought about it sooner and attack the one city they had on the Balkans before the second plague. I didnt expect it coming, and during 100 years two plagues hit me that killed 90 procent of my army and over 60 procent of my population twice! This meant, that I didnt have enough time to build new units and conquer the ottoman city before 1526. The conclusion is that it is not impossible, you just have to think about it sooner.
Thats where I finished the game, not going further. The third UHV may be possible, I didnt go that far, but what I can say is that I played the game with Spain until 1700 and never seen anyone adopting free religion despite having the technology. So it may be a matter of luck, the one sure thing is that on emperor you will fall behind in tech race, so you should just hope that no one adopts it and concentrate your later research on reaching liberalism asap or building the wonder that opens all religions.
Pros and cons of emperor game with hungary:
+ diplomatic relations were much harder to maintain, there were many wars between me, bulgaria, poland, austria, even with the norse. Also in general I had worse relations with everyone as on monarch difficulty, making trade more difficult
+ war with barbarians forces made the game really interesting and challenging, I recommend implementing barbarian AI and numbers from emperor to the monarch game or at least making the monarch barbarians stronger
+ stability - that was a real challenge, building manor houses and courthouses in every city is a must, but still I had to watch my stability all the time to not letting my empire collapse. Pure fun

- other civilizations war AI still incerdibly weak, only attack on me was from Austria, thanks to the fact Wien was just few tiles away from Trencsen and they sent a few units - dealt with them easily. Despite the more difficult diplomatic relations and wars I didnt see the difference between monarch and emperor, they just dont attack - where is the fun in that?
- plagues - I defended the plague system in my previous post, but now I've truly seen the difference between getting hit by a plague in a fully developed monarch difficulty Spain, where every city was very healthy and had plenty of health resources and between a less developed hungary on emperor difficulty, with weaker access to health resources. With Spain I didnt lose more than 20-30 procent of my population, with Hungary it was at least twice that much. This wouldnt bother me so much as losing all my army with every plague - That is something that makes playing with a militarily oriented civilization really frustrating.
- I think we have too much buildings with high production costs with low effects. I would propose either reducing the number of buildings (for example no need for smokehouse, keeping granary with 50 procent of food storage, etc), or reducing the production costs for all buildings by 10 procent.
+- tech race impossible, although big impact on my resarch had a military oriented economy. I rather see this as a challenge, not an issue, but maybe the plague not killing off all my army all the time would give a bit more time to develop my cities as well.