How hard is Kingmakers?
Harder than the trailers look, according to the game director. He says armoured enemies are not bullet sponges, that higher difficulty levels add more and tougher enemies instead, and that a man alone with an assault rifle loses to thirty archers. The developers have not published a list of difficulty levels, only the director’s remarks below.
You are meant to fight alongside your armies, and later missions and harder levels do not let you solo everything. What would change this: a list of difficulty levels, or the Steam page describing them.

What a player asked, and the answer
A player who watched the Gameplay Deep Dive worried that soldiers in the footage survive several bullets, when medieval armour would not stop rifle rounds. The game director’s reply began with the point about sponges:
“Look, the game isn’t about bullet sponge enemies. Higher difficulty levels create more enemies and harder enemies, not bullet sponges.”
So higher levels are about more enemies and tougher ones, not about each enemy soaking up more bullets. What the levels are called, how many there are, and whether every one is open from the start has not been said. He did say the game would come with “multiple polished missions and difficulty tiers for those missions”, in a post written as the first Early Access date was being called off. The one visible trace is the mission cards in the Deep Dive, each with a difficulty setting that reads Normal; they are transcribed on the Kingmakers campaign map.
Why the armour stops some bullets
Officers get better kit
“As you rack up victories, the enemy will promote more powerful officers.”
Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive
The narration goes on to say they are often given thick, heavy iron armour and shields that can deflect lower-caliber bullets, and that “So, weapon upgrades are crucial.” How officers work in a fight is on Kingmakers units and officers.
Where the tech comes from
“The English do receive limited help from adversaries with advanced tech. This help is 95% about enhancing existing medieval weapons and armor.”
“So they don’t just suddenly become modern soldiers. They become medieval soldiers with improved alloys that make their armor able to block lower caliber bullets.”
That is his explanation for armour that can block lower-caliber bullets. It does not name the adversaries; the setting is on Kingmakers story and setting.
Four ways through it
The same reply ends the armour question with a list, and the list is the closest thing to a guide the developers have given:
“So you can use explosives, higher caliber weapons, aim at weak points in their armor, or knock off pieces of their armor.”
“So they are really never bullet sponges, but you might have to use more effective weapons to get the penetration you need.”

Later on, you cannot solo everything

The rest of the reply is aimed at the idea that the trailers make the game look easy:
“I think people get the impression from the trailers that the game is very easy, but it isn’t. A man by himself with an assault rifle loses against 30 archers.”
“You do often need to fight with your armies to pull aggro off of yourself. Maybe not at the very start, but on harder difficulties and later missions, you really can’t solo everything even with air support and turrets and landmines and drones. You will just get overwhelmed by thousands of enemy soldiers.”
He made the same point earlier, in a thread about modern weapons against medieval armies:
“Even with airstrikes and modern weapons you’d be surprised how hard it is to fight against 1000s of soldiers, especially archers.”
“Most games never actually have you fight at that kind of scale.”
The video puts the odds the same way: “Even with an army at your side, you’re still heavily outnumbered.” Your side of that is the army you recruit, covered on Kingmakers units and officers, the walls and towers on Kingmakers base building, and the other players in Kingmakers co-op and multiplayer, where each player commands an army of their own.
Action or strategy, your call
In a separate thread, where a player guessed that the action is only about 20% of the playtime, the director wrote:
“You can freely move back and forth between action and issuing orders basically always.”
“Both playstyles work. Both sides scale up as you play.”
How the two views work together is on Kingmakers gameplay explained.