Most of what Tolkien wrote cannot be translated well onto the screen. It was why they dumped Tom Bombadil, Saruman the many Colored, the Razing of the Shire, and a whole host of other bullshit from the books. It is why the fucking Silmarillion will never be made into any media form beyond the book. That having been said, the biggest issues with ROTK were the drawn out fucking ending and the fact that the whole siege of Gondor felt too much like the Helms Deep shit from the previous movie.
Plus as much as I have enjoyed those films over the years, they suffer from the lack of a tangible villian most of the time. A lot of recent movies have the same issue, which is why despite the flaws movies like Dark Knight and Infinity War resonate with people and hold up. Turns out when you have a well crafted bad guy with a good actor behind them your movie gets shit tons better. Most action and sci-fi movies that end up on people's all time lists tend to have well crafted and acted (not necessarily powerful) antagonists, often with relatable goals. Movies like Fury Road, Dark Knight, Wrath of Khan, Die Hard (1 and 3 anyhow), and Blade Runner (the OG) hold up because their bad guys were as integral to the story as the protagonists. Most movies these days fail to flesh out the antagonist very well at all, which is coincidentally a big part of why the more recent Marvel offerings have been so meh.
Tolkien stories, outside of the Hobbit up until the point where Smoag gets gacked, all have nameless imperceptible antagonists that barely show up in the stories. It is a great story telling method for oral exposition and written story telling (which is most of what Tolkien intended with his works), as you let people's own imagination fill in the gaps with fear. But it does not translate for shit in live action media. PJ tied to address this a little by letting people see Sauron cut loose with the hammer, but it is just not as good or memorable as the kind of exchanges we got between Ledger Joker and Batman, between Ruger Hauer and Ford in OG Bladerunner, or more recently between Thanos and the various Avenger core. I could blame it on the recent rise of wokeism/PC shit in entertainment, but the trend has been ongoing for a long time in most forms of storytelling.
