First problem is input lag. It alone makes precision impossible, because you can't say whether your click was registered by game or not. Given huge problems with positioning and interaction, and that most combats being "two fast clicks" oriented, it all makes fighting quite frustrating.
When you need to position Geralt properly to interact with specific object (loot container, activate something, or talk to someone), simply turning Geralt towards something is a problem - game has zero reaction on quick tap on movement key. You have to hold it. And if you hold it too long, Geralt will start moving that direction. You were watching abyss? Hey, hello ground, long time no see!
On keyboards we have only 8 possible directions, available through keys. For more precision we have to adjust direction with mouse. Given narrow and non-adjustable field of view, plus glitches in lock-on function, this makes precision-required dodging into mouse jerk-fest. No, I don't have motion sickness, but I sick from doing that.
I know, I'm exaggerating a bit, but most enemies should be fought with "double-tap" approach, because third attack will be interrupted somehow. So we have to do two fast attacks, then dodge, or roll. Then two fast attacks again, then roll away again. This is some kind of rolling woody woodpecker simulator. Or rolling bomberman, if you don't like to keep things up close and personal, as many enemies, especially on higher difficulties, kill Geralt too fast (and I don't mind that, mutant or not, he is still human). So, even if you don't use signs and alchemy (I wonder when someone will beat Witcher 3 on highest difficulty using wheel), battles still feel the same, monotone, dull, boring. Game about best swordsman of North has boring fights. I'm PC gamer, I don't care for "wow-factor", I want consistent, enjoyable, comfortable controls and gameplay.
In addition, game has tonnes of micromanagement - oils and potions before fight, bombs and signs during the fight, no more than two consecutive attacks, fight after fight, hour after hour. Because of new, short potions' duration, and limited supply of potions and bombs, you have to pause to meditate frequently. No macros, no way to autodilute potions while on the go, since you can meditate for a minute and still replenish them. Those are not the droids I've been looking for.
To each his own, of course.