I am just about to start the DLC, but I think the quality of the major bosses in the base game is pretty good. I would actually say that my main complaint is that they aren't balanced around using all the new bullshit, so it ends up being more fun to fight them without it. Like, I'd love to mess around with spirit summons, but most of the bosses have terrible decision making when there's more than one target available, so you just get a bunch of free hits. For my replay I just used a bastard sword and restricted myself to "fair" Ashes of War that feel like normal parts of a weapon moveset, like Stamp (Sweep) and Impaling Thrust, and only used a shield on fights where I couldn't figure out how to do the fight without one, like Radahn, and it was a lot of fun! I also tried out perfumes for the first time and I think they might have to be relegated to the "overpowered" section in future playthroughs: Uplifting Aromatic in particular giving on-demand Opaline Bubbletear effect is extremely powerful and let me win some fights that I wasn't actually proficient in. (Also it's not especially fun to farm snow trolls for Arteria Leaf once you run out.)

My secondary complaint is that a lot of minor bosses, especially in the overworld, have a lot of terrain jank and camera issues, like Lansseax going down the nearby cliff and then having weird geometry issues with the tiny raised section down there, or the tree spirit "miniboss" in the rotten pond in the Haligtree that is encircled by raised roots that make the proper part of the boss arena too small, and which it regularly clips through during its animations, making it even harder to see what it's doing than usual.