I wish I could give this game a positive review, but there are some very important things that prevent me from doing so.
How supply works is that there is a supply cap on certain provinces, and the supply cap can only support armies of equivalent size. However, when you get a decently sized army going, it is near impossible to find provinces able to support your entire army size, so you will need to split your army once, even twice, and spread out between different tiles so that your individual armies are supported by the low supply caps. And with how quickly supply gets used by your army, you will need to do this a lot. If you don't do this, you will start losing certain percentages of your army due to attrition. Large percentages. And you will sometimes run out of supply in the middle of sieges in enemy territory, causing you to need to stop the siege and subsequently all of its progress, and go back to friendly territory to get supply so you don't lose half of your army.
So, you might think, the answer to this is to just have your army split at all times with men-at-arms evenly divided between all of them. But this is a terrible idea, because of a little think called "death-stacking." The enemy will always consolidate their troops into a single mega-army, and if one of your army groups gets attacked by it, it starts taking casualties almost immediately, causing any reinforcing armies to be weaker when compared to the enemy as a result, because of the casualties already taken. Thus, it is nearly imperative to keep your troops in a single death-stack mega-army, otherwise you will die.
I also have a problem with how victory mechanics for battles work. When you win a battle, the enemy army gets routed, keeping a portion of their army and retreating from your attack. This can lead to extremely tedious situations, because normally you can't just pursue them into their territory for one big reason. If you go too deep into an enemy's territory without having captured territory by sieging forts along the way, you will lose a portion of your army walking through the enemy territory. This is generally not advised, also because you will need to eventually get out of the enemy territory to get more supply, causing you to take double of the casualties from this cause. Now, you might be thinking "OK all I need to do is defeat the enemy two or three times to demolish their army" but no, this is completely wrong. Sometimes you will need to defeat them 10 times to fully destroy their army. And if you just decide that destroying their army isn't worth it, they will CONSTANTLY undo the sieging you did in their territory, or they will siege your own territory, in the most tedious exercise of essentially whack-a-mole that you will ever see.
If you decide to expand your empire by declaring war on someone, other AI will rightfully see you to be in a weaker state given that you're committing troops to a war. If an AI decides to go to war with you while you're already in one, if you're lucky, it will be an outside empire. And there will be nothing you can do when they start pillaging your empire from across the map and collecting war score. When you finally finish your war, there is a good chance you will need to sign a white peace for two reasons: 1. it might be literally impossible to get enough war score to "enforce your demands" (automatically win the war with enough war score) 2. you will be in debt and your empire and troops will hate you because of how much money the army upkeep is in the game. Now, if you're UNLUCKY, you will need to beware of internal threats looking to dissolve your empire, overthrow you, or get independence, coming from your very own lovely vassals. And usually, your vassals, when they go to war with you like this, are much stronger than you in their combined strength, which means you will almost guaranteed lose.
Vassals will join factions against you like the independence, dissolution, and overthrowing ones I mentioned previously. Whether a vassal joins your faction or not is dependent on a number, either positive or negative, with modifiers for either. One of the positive modifiers for willingness to join a faction is the vassal being not in your de jure empire, kingdom, or duchy, (essentially you are not their rightful liege). And this modifier is MASSIVE and impossible to counteract without you becoming their de jure liege. However, you can only have a set number of vassals. And when you start consolidating your vassals' titles into duchies, then kingdoms, they will be more likely to want things from you because they will be more powerful. Vassals who own kingdoms in your empire will be more likely to want independence and you can ask of them less because they are kings/queens. This is extremely annoying for the stability of your empire. For most of the game, until you get one of the late-game cultural ideas "primogeniture," you will only be able to have one de jure empire title, for fear of your empire being split in two when you die as the titles will be split between your heirs, due to how succession laws work before primogeniture. Thus, you will not be able to have really any vassals with kingdoms outside of your de jure empire title, because they will want independence almost immediately because you are not their de jure liege. When you get the primogeniture cultural idea, that's not the end of the story for this. To unlock the succession laws that allow for only one heir at a time with primogeniture, you will need to increase your "crown authority" over your vassals. This gives you a bunch of benefits, like increased troop and tax contribution from your vassals. Increasing your crown authority can only happen once every certain number of years (I forget the amount but it's quite a while), and in that time, your vassals will NOT be happy at all with your choice to increase crown authority. In fact, there is a whole faction that can be formed against you for the sole purpose of lowering your crown authority. You need to get to crown authority 4, starting at 1, to be able to change succession laws. Primogeniture is a late-game cultural idea, so you will either need to have very good timing on your cultural idea progress, start raising crown authority early, or do crown authority when you get primogeniture and have it take a long-ass time. You will also need all of your powerful vassals to have high opinion of you to change succession laws, which, when you have crown authority 4, is kind of hard (though certainly not impossible).
My final big issue is legitimacy. Legitimacy determines how legitimate of a ruler of your county, duchy, kingdom, or empire you are. Low legitimacy gives A BUNCH of negative effects, the most annoying being low vassal opinion of you, making them likelier to join factions against you. Legitimacy is very hard to gain. You gain legitimacy by hosting/going to events like hunts/feasts/pilgrimages which cost a lot of money and honestly give a pitiful amount of legitimacy, or by holding court. These events have a cooldown of a few years for each respective event after hosting one. You can only hold court once a year. The amount of legitimacy these two categories give you is abysmal when compared to how much legitimacy you need for having, say, an empire. It's also pretty much a cointoss in my experience if your heirs will have high or low legitimacy when you die, making it frustrating when you become your heir, have low legitimacy, and your whole realm falls apart because everyone wants to join factions against you due to your low legitimacy. There might be a mechanic to give heirs more legitimacy but I do not know of it. Regardless, legitimacy is one of the worst mechanics I've ever seen.