After playing some more, I think the core issue with KCD's melee combat is that it doesn't utilize its own strengths properly. Medieval combat (or HEMA) is centered around alternating between reading your opponent and reacting to what he does, and taking initiative and trying to attack him in a deceptive way.
Many years ago, Gothic 1 already implemented an extremely simple model of this, with its block and counter-attack mechanics. This was then re-implemented in many games since, including Gothic 2, Risen, and Witcher 3.
KCD has a much much more complex combat model, and theoretically, it could support a vastly in-depth fencing model. But for whatever reason, Warhorse chose to ignore most of the complexity, and roll out a much simpler final product that is not much different from those other games. Limited funds? Afraid to scare off casuals?
For instance, they already have the directional model in the game, for attacks, stances, etc, and it would be fairly easy to introduce directional blocking. Instead, they chose to go with a dumbed down one button block, which requires zero strategy or technique, and is the same as many other, much simpler games.
Another example, they create this fancy directional attack model, but what is the use of it exactly? They could have made it so that depending on the enemy's position and guard, certain areas would be more vulnerable, but in my testing, I did not see any of that. Regardless of the direction, attacks were blocked most of the time, and only random chance and character stats seemed to play a role. Even feints were mostly blocked. Combos use directional attacks, yes, but combos are a fairly useless feature, since any competent opponent will interrupt them.
Then we get to stuff like masterstrikes and slo-mo, which completely eliminate player skill from the equation and ruin the flow of combat. With this feature, KCD's combat is pretty much like Witcher 3 combat against humans.
I guess the positive thing is that all of this might be fixable by mods, we will see.
