Before I go too far, I want to clarify that I don't think the Frieren anime as an overall experience is bad, as reflected by my score. I think it's fine for shows to have both things that are very good and things that are not so good, and especially for something as highly acclaimed as Frieren I believe it's fair to point out the less shiny parts whenever you see them.

I'll start with the good points, and that's basically everything related to the production. Madhouse did a phenomenal job adapting this.
Frieren has both great action scenes and great slice of life scenes thanks to very solid character design and art, and clip-worthy animation through its entire run.
The OST is also a crucial part of this, as it elevates both the action and the not-so-action scenes creating the perfect atmosphere.
Especially in the first half, we were also shown a lot of detailed and generally very good looking sceneries.
In short, from a production standpoint, this anime did everything it possibly could to elevate the source material.

Now that was pretty much all of the good. The issue I had with this show was the substance. Through all of this first season, Frieren felt like one of the most backwards experiences I had watching anime.
"Philosophical" shows usually go like this: you have stuff going on, the characters go through experiences while dealing with it, and as they do you're able to formulate your own thoughts about what they're going through, you try to interpret the messages, have your own takeaways and so on.
With Frieren it's the complete opposite: stuff happens just for the sake of having those takeaways and slapping the message to your face. It's something that happens throughout the whole 28 episodes, and it gets so repetitive that I started wondering if the series simply assumes its audience isn’t smart enough to understand what the show is about unless it was reiterated to them every couple episodes.

The result of this storytelling approach is that the show has no real plot for the entire first half: it's a collection of borderline irrelevant events leading to parallels between the present and the past, with barely any differences in what they're trying to tell us or make us feel.
Frieren masquerades as a deep and thought-provoking series with a poignant message as it baits the viewers with a fake deep approach to give them a false sense of accomplishment as can say "Wow, I've finally understood everything about an esoteric anime", while in truth everyone gets the message since it's plastered to your face at every occasion.

The second half of Frieren does actually have a plot, but it still didn't manage to impress me as it feels pretty irrelevant to what the show has been so devotedly been telling us it's about. Does anything about this exam arc hold any relevance to Frieren's understanding of humans? The arc pretty much exists to show us how cool Frieren is as everyone is taken by surprise (me too, I'm surprised they're surprised. She is literally a legendary hero but barely anyone knows about her... aren't mages pretty diligent in their studies in most fantasy settings?).

To complement the plot (when there is one), we have a main cast made of three characters: Fern the Pouty, Frieren the Cool and Stark (just Stark, it's not his fault).
As the name suggests, Fern pouts. That's about it.
Frieren is very cool, she barely ever shows any engagement with whatever is going on in the present but makes a cool "oh" to a "woah" whenever she reminisces about the past.
Stark was a very interesting character when he was introduced, the parallels with Eisen were actually hitting me pretty well and his whole vibe felt like a real person’s. Unfortunately the focus shifts much more to the two girls in the second half so we don't get as much on him.

The way the characters interact also gets tremendously monotonous, a stale compilation of misunderstanding leading them to figure out the other person just to proceed to have another misunderstanding in the following episode.

Giving credit where it's due, there are some worldbuilding elements I found interesting, for example the evolution of magic over time and the nature of demons in this series, and some of the side characters were actually interesting, especially Frieren’s former party and Sein.

To conclude, Frieren the anime takes a monotone, overly simplified and overall mediocre base and polishes it to the extreme through the efforts of everyone involved in its production. It's a show that can go from extremely entertaining through the sheer power of animation to extremely frustrating in its monotony.
Truly So-so no Frieren.