I think this goes back to Slider_Quinn21's point: when a story leaves the audience satisfied with the characters' journeys, then it's alright for the Mystery Box content to be disjointed, contradictory, unresolved, and not even that interesting and often out of focus. The audience will forgive that. Or at least Slider_Quinn21 will. Again, I have not finished LOST, but my sense from Slider_Quinn21 is that even if the Island wasn't ever explained, the character arcs were satisfying.
I think this is a fair statement.
Were all of LOST's answers satisfying? No. Were all of them reasonable? No. Were some of them, after watching the show multiple times, confusing and/or dumb? Absolutely.
But mysteries are hard. You have to come up with a mystery that is interesting enough to keep an audience interested that is a) mysterious enough that the audience won't figure it out and b) simple enough that it's satisfying upon rewatch and c) with enough breadcrumbs that your audience won't quit because you aren't revealing anything.
LOST set up some simple mysteries. What is up with this island? What is this monster that keeps killing people? Who are these "Others" on the Island?
The show couldn't really explain the first one since it was the primary question. So it solved the question about the Others. In season one, we knew one Other who died before he could reveal anything (Ethan). In season two, we met another, but he kept his identity secret. By the time he revealed who he was, his answers very vague and, well, mysterious. But by season three, we were spending time with the Others. By season 5, we basically knew everything we needed to know about them.
The writers knew this was difficult. That's why they insisted the show end after six seasons - they didn't have enough ways to delay gratification. You can answer the Others question but you have to open up other mysteries.
As far as what the Island ended up being...to me, it didn't matter. The answer is still vague to me. The Island has some sort of cosmic importance, but it's unknown what the Island being destroyed would mean for anyone. It ends up becoming a McGuffin. But by that point, the characters were so beloved (to me and others) that it didn't really matter to us. The Island could've been a cosmic turtle or purgatory or the town from the Village and I wouldn't care. It didn't magically make those answers more satisfying - it made the mysteries less important to me.
So that's why I understand why people get mad at LOST for the mystery aspect. But that means that they never bought into the character aspect. Which, again, is fine, but that means they essentially wasted half of every episode getting to know characters they didn't have any interest in getting to know.