The DLC makes it harder to build your character aesthetic around different themes

I feel like the most fun I've had in the game is when I didn't care about getting the absolutely biggest buff and all the damage bonus percentages right.

I want my armor pieces to fit aesthetically, so if I'm going for a death knight I want death knight-themed abilities, weapons and clothes. Or a rot sorcerer - then I make myself look all decrepit and put on mushroom clothes and get scarlet rot-based spells and maybe the scorpion dagger.

In the base game you could get away with a lot more and it was very fun.

In the DLC enemies attack more relentlessly with longer combos and it just makes it very hard to use anything that has a longer cast time. it makes it harder to also use sub-optimal builds.

It's worst at the DLC bosses where longer casting spells or incantations are out of the question, you need to just poke them real fast in the short windows they give you between their massive combos.
So everything becomes more of a poke fest. Of course if you are extremely skilled, you can get away with more, but for most players, it will funnel them into a more narrow set of builds that revolve around managing the very short windows of opportunity to attack a boss between their relentless chained attacks.

And it's a shame because there are a bunch of new cool looking spells, weapons and incantations.

Can I realistically use the new rot butterflies in most situations? Not really. And the Light of Miquella, the god-like ability that we get after defeating a god, I am not sure what it should even be used for. The cast time is way too long and leaves you extremely vulnerable. In every case I'd be 100 times better off to just poke enemies with a rapier.
Speed beats everything in the DLC, the knight's lightning spear due to its cast speed is always 1000 times more useful than the supposed god-like ability you get after defeating the final boss.