I'll quote a comment from @Eldy Dunami that prompted the idea for this thread:
Tolkien drew a ton of inspiration from historical literary and mythological works, of course, but his treatment of industrialism (most things related to Sauron and Saruman), degradation of nature, colonialism (Númenor), the horrors of industrial warfare (the original version of the Fall of Gondolin), and other themes clearly mark his work as the product of a modern—specifically, 20th century—author. That's not a criticism; I think all art reflects the context its creator(s) lived in. Adaptations are no exception, so I'm not bothered by ROP being recognizably the product of 21st century writers. (bolding my emphasis)
That got me thinking (beyond the initial thought of "that's a good point") "all art reflects the context its creator(s) lived in." I agree that it's no different for adaptations, so I'm not surprised creators of adaptations and writers of fanfiction, inspired by Tolkien, will also have other influences when creating art. Yes, if you create Tolkien fanfiction, Tolkien is probably your largest influence, but like Tolkien, you are influenced by many factors. Readers, fans, critics of the art also bring their own experiences that color their opinions when reading any adaptation. I guess the questions I want to ask are really loaded, but perhaps we can put our brains together and get a few theories going.
What makes a "good" adaptation? What makes an adaptation "bad"? All adaptations change parts of the canon of what they're adapting. What makes one change to the canon more "acceptable" than others? How many "changes" to the canon are acceptable before you wonder can this even be called an adaptation? Is there some sort of scale? Like are some changes worse than others? And I'm just predicting there will be a variety of answers to these questions.
One final thing, that also prompted my thoughts. I enjoyed the Bakshi Lord of the Rings adaptation, even pantsless Aragorn and Viking Boromir never really bothered me. In Jackson's Lord of the Rings, I accepted a waffly Aragorn and a softer (plus strawberry-blonde wigged!) Boromir, because it made sense for the story Jackson was telling. Those characters wouldn't have worked in Tolkien's books, but the changes worked in the films, because it was consistent to their own setting. This is of course, all my opinion and I expect others will disagree. I had not seen the Rankin/Bass Hobbit adaptation and when I watched it recently, I really did not like it. I was quite bothered by the changes to the canon. I'll give it credit for wanting to make the story about Bilbo, but I particularly didn't like the conclusion. It was quite a poor conclusion (basically Gandalf says 8 of the dwarves died at the battle, but the only ones we know are Thorin and Bombur. We are left with no idea which dwarves died, because there's no scene of Bilbo with the surviving dwarves at the end. It's just Gandalf tells us 8 dwarves died, that's it. Bilbo goes home.) And it got me thinking why did that change bother me more than say a waffly-Aragorn did? I think it just comes down to good storytelling vs. bad storytelling (in my opinion). It's not the changes that bother me, it's wanting to have an explanation for why an adaptation changed something. And if you are going to change something, explain it better so it's consistent with the story you're trying to tell.