I have played a few high elf and empire campaigns on Standard. I would like to offer more feedback on easy, not on legendary as I am not that skilled.
From what I can see most of the update is good. I like most of the changes and how the higher difficulty is longer. it doesn't seem like the AI is cheating as much when they randomly get an army of higher grade units that doesn't make sense on how they got those troops. It sucked when you fought against 4 or 5 armies only for another 5 to march on you a few turns later. Using those low tier units for longer is great as it becomes stale when you have nothing to look forward too.
I'll go on building 1st. I think its a nice change as the cost of getting to a point isn't great but to have those great bastions of your empire is going to be costly so having a level 5 city means a lot. The cost of those epic, and iconic landmarks is also good as when you do get it, like the Imperial palace it feels good but some of those landmarks lost some of their worth with the update and changing some of their effects may be nice.
I like how the units cost more, it means you must really think about what your going to get and what to do with them. Going into a battle that costs your army at the death of an enemy force as the price to rebuild is much greater. Do you combine those too troops and save a turn of healing or save the money? It's a good question now that I had to think about, same with disbanding an army to try and build faster. Those types of choices are great.
I do however have a few issues that I will go over, mainly with the snowball effect. The game I would say is harder and more random which is good however once you start really building your empire and get to the point where your mass is so large nothing can defeat it the same issues from before remain. I would look at making larger empires having greater debuffs with this release. I can think of a few ways. 1st is an greater exponential increase of upkeep for every army like the old supplylines but greater as you get more troops. so that when you have only 3 or 5 it doesn't change a lot but over 10 it starts to really stack up. There could also be a increase of upkeep for like units in the advanced military buildings. IE you don't have to worry about it with spearmen however every unit of swordmasters increase the upkeep by like 2%. So 20 units across 10 armies wouldn't be bad at just another 40% increase however if you doomstack mammoths and have 100 of them it starts to really hurt. You can do a small percent of advanced units that aren't common but hard to train but have dragons and alike being a lot higher for every one.
Another method is to look at people turning against you for getting very large (a better version that just keeps getting worse for the large factions so when you own half the world the other half is going to be very worried) or rapid expansion defuff in diplomacy.
I know this doesn't fit the warhammer world too much (save for the plot armor parts) but I would really like to see some way to get LL after their faction is defeated. Like Marcus Wolfheart got beat up in Lustra you have a random event that pops up that gives you 20 turns either send a hero half way across the sea to find a lone boat that survived with him and his heroes or spend 10000 gold or something. with the new randomness to the game you'll find more LL defeated along the way however I've always been something of a collector in these games and would like something in say non legendary to be able to save people without me sending a hero halfway a cross the world, only to bribe, gift and declare war on a butch of people who can never fight me, only to get Marcus to join me. It makes starting some campaigns really poor.
Overall I am loving this game, great work. Looking forward to the next DLC.