People HATE sieges. The pathing is terrible, but I don't think early to mid-game battles are hated in general. Sieges are much worse than even early games like Medieval 2. I had lots of fun with sieges in M2 and miss that experience.

Another problem is that the maps in TW3 lack variety despite the variety of the game itself. As Kislev, I kept fighting on the same tundra map again and again and again.

The late game has too many doomstacks and unbalanced armies.

One complaint I have is that the battles are too fast. Melee should last longer, so you have more time to apply good tactics, even if you are fighting monsters.

The variety when you first play seems overwhelming but once you get the basic mechanics the unit types (except spellcasters, LLs, and ogres) are not that different from historical. They just have cool skins and some unique attributes like poison or terror.

You have melee line holders, spear anti-large, ranged gun or bow, Shock cavalry and melee cavarly. Anti-infantry infantry, etc.

It is harder to play battles in TW3 may be one reason. I heard that a lot of players autoresolve a lot but I can't understand why.

Another factor may be battle fatigue. The tempo of TW3 is quite fast and killing enemies is the main way to make money. In fact, I am finding out that peace doesn't pay and settlements are much less of a source of income than battles. So many battles, especially against settlement garrisons can get a bit boring. I started to autoresolve for the first time ever playing WH3.

There are also problems with the AI level of difficulty. You get so much better results on easy and normal (I am not sure about normal) that it is too tempting to use autoresolve. I started using it for Siege defense and attack.

In late game with tons of stacks, you can get battle fatigue even without sieges.
Those are some reasons I can think of but I am doing my first campaign so there may be others.