Getting to the point: I absolutely despise the final boss fight, but love basically every other single boss in the DLC. Specifically, the delayed AoE adds a lot of visual noise and makes it way more difficult to learn the dodge patterns for the moves. Once you do know the pattern, it's not that much tougher than the other remembrance fights, but it specifically punishes many builds (spells, heavy armor, big weapons) and is literally the only fight in the game that I couldn't visually parse and learn on my own.

I have been following the chat about this DLC for over a week in 2 different Discords as well as watching several streamers, and I have heard one person say they once heard someone else say they kind of like the final boss, but other than that, literally everyone who has seen the fight hates it. The thing that makes this stand out so much is that there are so many other bosses that people absolutely love that could be the final boss instead without any changes:
-Leda and Co: The culmination of every NPC questline, a difficult fight where you have to really learn the moveset of all of your friends, and the difficulty is based on how many questlines you finished/howany people you ignored? That would be an INCREDIBLE ending and something they haven't done before! A proxy for each of the demigods shouts their ideology at each other as everyone is in a fast-paced, chaotic battle showing off many neat character builds you can do! "Top 10 Anime Moments" in the best possible way, and then all of the lore heads get to go "Whoa, deep, the cycle of violence continues even after everyone in The Shattering is long dead".
-Bayle: Dragon Moby Dick with one of the best hype men I have ever seen monologuing during the fight, as well as really cool lore and design relating to Placidusax, another really cool boss from the base game. There's a dragon head biting his shoulder! He's so cool!
-Messmer: just as fast and chaotic as Radahn, but WAY easier to parse what's happening and learn the timings for moves. He's the character they made a statue of and has half of Rykard's moves, which is another very popular boss from the base game!
-Mother of Fingers: The base game (most likely) ends with you rejecting the Two Fingers in some way, so fighting the fucked up monstrosity origin of them is a cool, thematically appropriate way to end the DLC. Fighting Count Ymir afterwards as a sad, pathetic man trying to use power that you already have seen is fucked up and leads to bad things would also be a great capstone to the whole thing.
-Midra, the wannabe Lord of Frenzied Flame: People playing the DLC almost certainly know that becoming The Lord of Frenzied Flame is one of the base game endings, so either your reaction is "Oh shit, we have to stop him or chaos will take the world!" Or "This town ain't big enough for the two of us! *I* am the Lord of Frenzied Flame!".

The exploration and level design is also nonstop bangers:
-When you get ganked by a curseblade after walking 10 feet into the DLC and say "We're so back!"
-When you see your first Wickerman and it attacks you is an amazing "Oh shit!" moment
-Blackgaol Knight being a tough Guts-like boss in the very beginning that gives you a bunch of cool stuff is a fantastic warmup and first boss of the DLC.
-Walking into the Abyssal Woods and thinking "Heehee I'm in danger!"
-Walking into the Finger Ruins and it's even more fucked up than you expected
-The 5 times I looked something up and found there's ANOTHER part of Shadowkeep that I hadn't been to yet
-Walking into Rauh Ruins and being awestruck at how cool and different it looks
-Etc