Firstly, I don't want to stir up controversy, Sou Sou no Frieren is an okay-ish anime with “many” good points. However, there are many other anime that are significantly better than Sou Sou no Frieren, and here are the reasons why.
1. Boring Characters
Frieren is an intriguing elf with a long lifespan who doesn’t really value her time, as you can see in some episodes. Eventually, after people she knows start to pass away, she begins to rethink her life. However, she doesn’t reveal much about herself, and so on throughout the story, this becomes quite boring. The other characters in the anime aren’t much better. They are totally forgettable and nor do they change or develop, making it hard to get invested in them. It feels like they are just there, not adding much to the overall experience.
Stark & Fern
Tbh, I don’t really understand why Stark and Fern are so popular. They feel like filler characters who make the story dull. All they do is fight over the stupidest things, which eventually becomes annoying.
2. Non-existent plot and story
The story moves as fast as watching grass grow. The journey starts after the hero’s party defeats the demon king and parts ways, which is at first interesting since a lot of anime take the opposite approach. Instead It’s filled with dull, repetitive stuff, as you keep watching and hoping for something "interesting" to happen (Spoiler Alert it doesn't). No surprises, no excitement, no nothing. Personally I think that Sou Sou no frieren lacks the WOW factor like any other anime. From the beginning to the end, around 20 eps-ish that make me feel like I'm watching an high rated anime but still it does not compare to other great anime.
3. The Pacing
As I mentioned before, the pacing is all over the place. Some parts drag on endlessly (like spending 2-3 eps just to find a flower to put on an old statue), while others are rushed through without giving any time to understand the characters (for example, the background of Frieren and her friends like I still don't really know 'til this point). This inconsistency makes it difficult to enjoy or feel profoundly engaged with the story.
In summary, Sou Sou no Frieren is a huge disappointment and a waste of time. My BIGGEST ADVICE is to save your time and invest it in something that actually delivers on its promise.