Personally, I love Frieren for how it handles long-lived characters, for both good and ill.
All too often in fiction they're either made to look incompetent by having shorter-lived beings match their skill with only a couple decades of experience, or the story just devolves into a soap-box about how perfect and superior they are.
But Frieren manages to strike a perfect balance by not shying away from showing how much power an elf or a demon can amass with centuries and millennia of study and practice, which humans simply cannot replicate. But that doesn't mean they can't lose to a bad matchup or because magical theory has advanced while they were left behind, because raw power isn't everything, and the story also showcases how their age works against them: to an elf or a demon, a spell that was developed a mere eighty years ago is still brand-new and can still take them off-guard, while humans have known it their whole lives as the cornerstone of magical theory, and have become familiar with it in ways longer-lived beings simply cannot. If you learned a bad habit a thousand years ago it's practically impossible to unlearn now, even if it's an incredibly basic mistake that's normally corrected while you are still an apprentice. And so on.
It also shows in detail how elves struggle with connecting with people, becoming overcome with grief or denial when their companions pass away, and how they can procrastinate endlessly unless circumstances push them to take action simply because they can take decades and centuries to mull over a decision or obsessing over a single topic.
No spoilers, but in my opinion one of the most poignant lines from the entire story is "I've lived a long life, but I haven't done much with it.", and how it relates to the main elves featured in the story. What good is power that's not used for anything?
