The RPG mechanics just don't matter, that's the problem. No matter how you allocate your stats, it's still swish, swish, swish, Sign, swish, swish, swish. All the fights go the same way. At least in TW1 putting a pip into a tree got you a new move and you had to pay a little attention to the rhythm and sound to get it right. There was some feel of progression and your character-building choices making a difference. With TW3, nothing really changes.
Unlocking alternative modes for all signs? Gaining access to special attacks like Whirl or Rend? Being able to use cluster bombs? Using Adrenaline Points as an extension of Stamina and/or Vitality, or to increase damage? Being able to imbibe several powerful decoctions at once? Upgrading your potions and oils to have a much longer duration, five times the effectiveness of their base versions and possible other special effects? Multiplying your sign intensity or attack power with the right combination of abilities, mutagens and gear? I'd say there's a pretty clear sense of progression there.
CDPR clearly tried to make different enemies more distinct this time and make the combat more varied. There are some clever uses for signs, for example: using Axii on an alghoul to make it hide its spikes or a wolf to make it leave you alone, using Aard to put out fires (useful against fire elementals) or bring down flying opponents, using Yrden to trap incorporeal creatures, having Igni melt armor or send forth a continuous stream of fire rather than a projectile, and so on. Flying enemies require you to use Aard or a crossbow to knock them out of the sky, fiends can hypnotize you and severely impair your vision, wraiths can teleport around while also being incorporeal most of the time unless you use the appropriate sign or bomb, nekkers can be stunlocked while drowners can't, water hags will throw shit on your face and stun you, wolves always attack you in packs while bears are usually alone and use their size to run directly at you and possibly knock you down, arachasae are poisonous and can root you to your spot by spitting web-like stuff at you, sirens will start destroying your boat if you encounter them on sea, endgregas try to hit you with their long-reaching tail that you need to get around, getting close to a fire elemental without fire protection will get you burned...
In some ways it's a success, and different fights and builds may actually play in quite different ways. However, it's somewhat diminished by the fact that the enemy variety indeed leaves something to be desired, as certain enemy types are so prevalent and have a very wide level range, whereas some enemies only appear rarely, or only provide a challenge if you happen to encounter them when they're close to your level. Drowners, for instance, remain worthy opponents almost through the entire game, starting from around level 4 and ending up in the high 20's, but many of the more interesting enemy types have a much smaller range (not that it's in itself a bad thing) and become trivial pretty soon after you're able to finally kill them. A flatter power curve and the removal of the current level-based damage reduction system might've improved the combat a lot by keeping even lower-level mobs dangerous throughout the game and allowing you to get rid of stuff like level 22 ghouls altogether.