I'm almost at the end of the game, playing with "standard" combat system and "Equilibrate" difficulty, so these are my impressions:
First of all, the difficulty and the combat system can be changed on the go, they just need a reload of the game (or area) to become effective.
I didn't tried the "Classic" combat system, so I don't know exactly in what differs, I just know that in the previous games I was able to counter 100% of the time, while here the time window is really short, and miss a lot of times.
It is not so clear, for every enemy, when you have to hit the evasion to start a proper counter... some enemies have a colored highlight on the weapon, while others just adopt a particular position.
Also the same enemy could have different moves that require a different timing, but the blade is highlighted with the same color... so their position is really important to know what they are starting to do.
When an evasion is not exactly accurate, you will go back, but can be hit, while if the timing is right, a cinematic slow-down happens, making you invulnerable, and during this time-frame, you can issue a powerful counter, with area damage, while still invulnerable.
You can really beat a group of enemies without being hit if you evade perfectly every attack... but the camera view is not so helpfull when you are circled... at least there there are arrows that will point at the direction of the enemies that you can't see, and those will become red if an attack outside the view is coming.
The game is really designed with counter moves in mind... most of the bosses are too fast to be evaded normally, and get a too low damage from normal attacks... you just have to stay calm... wait... and EVADE+COUNTER !!
If this is done right, you can beat every boss standing still, without taking damage at all.
But as I told before, the time-window for the right evasion is really short.
There is anyway, later in the game, an artifact that can boost this time-frame.
In the worst case, just leveling up make you super strong... so the enemies you struggled to beat at the beginning, will die with just one hit at some point... and this is actually good, because you have to return to many previously visited areas during the story.