Hmm... trying to think back to the boss battles. Revenants, that dragon with the chalice, Morrigan's mom, the shit that went down in the Deep Roads... it's been a while since I played it but honestly? I don't really remember enjoying any of them much. It was either a matter of finding a simplistic tactic and applying it to win (e.g. the very first boss, the ogre in the tower -- kite it and have everyone shoot it to death, or the dragon with the chalice -- use that one spell that makes you invulnerable, slap it on your tank who draws aggro, then use the rest of the party to kill it).
I meant encounters like Jarvia (female dwarf mob boss, she feels the room with traps and often switches between ranged and stealth melee), Sloth Demon that has a variety of different forms (each with its own strengths and weaknesses), some ambushes like Cauthrien and Zevran's assassin pal, Gaxkang, the Demon boss in the Mage Tower that turns imprisoned mages into abominations during the course of the battle and switches between casting spells and melee attacks, Broodmother with tentacles and grabbing attacks etc.
Mind you, it's not just about challenge (we can all agree 99% of RPGs are relatively easy once you get a solid grasp of their systems) but set piece/boss fights throwing something new at you and standing out from trash fights leading up to it.
In PoE in vast majority of cases you were for example fighting 15 groups that all composed of a say fighter, mage, cypher and cleric and then faced a slightly tougher group of the same composition at the end as a boss fight, it felt repetitive.
Undead Raedric is a significant and materially different challenge, and some of the bounties are downright entertaining, as are some of the tougher levels of Od Nua and tougher encounters in White March 1.
Undead Raedric is a tough cookie (so is the Mind Flayer bounty fight, the rest of them not so much as far as I recall) but it's still something you faced before (vamps) just in a larger quantity (granted DeathKnights with Fear Aura /whatever they're called are pretty rare IIRC and Undead Raedric is a tougher version of one obviously).
I'd say my favourite fight in PoE is Thaos actually, a high level priest that can do a body switch with his statues that have a nasty ability to explode upon death definitely feels as a unique encounter compared to the rest. In that regard PoE certainly beats DAO and it's banal, shit, boring Archdemon by a landslide.
From where I'm at you're exaggerating a bit, but it's not entirely wrong. The main problem with the "sameyness" of fights was the AI -- it would just latch onto whoever was closest, and was not particularly aggressive or smart using its special abilities. This has changed dramatically with version 2.0 -- when it came out, there was a lot of whining that the Adra Dragon is now "impossible" for example. (They'd given it fire immunity and made it use its breath weapon aggressively, among other things. IOW it became a genuine challenge.)
Yeah, from what I followed of the post-release development I do think some of the changes/fixes sound promising, in particular the improving enemy target selection, ability usage and inclusion of immunities. I'll definitely do another run of the game down the road (probably after the 2nd expansion comes out).
That said, I never thought much of Adra Dragon fight, it's just a big monster that hits extremely hard compared to nearly everything else in the game, it's as one-dimensional as they come.
I far prefer Pillars' itemization. DA:O just had piles and piles of ever-more-epic loot with ever-higher numbers stacked on them. With Pillars, most unique items actually have some unique properties on them, and there are non-obvious ways of getting maximal advantage out of those properties. One major pro for Pillars is also that the usual linear progression isn't there: you get damn good items right from Act 1, so instead of just using them for a bit and then throwing them out for the next-greatest-thing, the challenge becomes finding the set of items with the unique properties that you want for the particular build you want to support, and, conversely, to optimally distribute the uniques you have at any given time.
DAO didn't exactly have linear item progression either, many of the most powerful items can be bough in shops or are acquired after solving puzzles (which granted are usually pretty simple) or doing side quests.
The key difference here is the lack of all-powerful crafting which means I'm motivated to explore to find good gear or raise money to buy it instead of just getting a regular version of the weapon my char's adept at using and then enchanting the shit out of it until it becomes a tool of mass destruction. Even in those rare instances the crafting is done right in PoE, such as collecting pieces of a legendary item and getting them to master smith (which one figures would be much better at it than a rag-tag party of adventurers) the results are disappointing to say the least.
Another difference is the accessory items like belts, rings, helmets etc. which are useful/significant in DAO but in PoE so bland and meaningless I could barely remember any of them right after finishing the game, you can't even stack +stat items to improve a char's weakness in a certain area. One exception are summoning items which are great and can be a game changer in tougher fights.
Some of the feel of "too many interchangeable uniques" comes from the fact that there really are many of them, and whatever your build or tactic happens to be, a lot of them won't be relevant to it. Trouble is, if you want to give enough uniques to support a big variety of builds and tactics, that's always going to be the case.
I actually missed the whole point of the Pillars itemisation system when I wrote the review here. I've only figured it out later, and it has seriously grown on me.
Quantity can be an issue (in low level games magic items stand out by the virtue of them being very rare) but I feel like quality is a bigger one in this instance. They just aren't memorable and impactful enough and they have to compete with an all-purpose crafting system (that yielded your rusty dagger of awesomeness). Even staunch criticizers of say BG2 (you can also use PST for example) itemization seem to know it's item selection by heart, that's hardly the case with PoE for a reason.
I get your point that some of them are apparently very effective/shine when combined with a specific build but I just didn't explore the game to that degree to experience it.