Background, animation and special effects look quite good, but I do not like character design, especially the fact that almost everyone has huge foreheads.
As for the story - meh. The high-level concept described in the very beginning seems interesting, but the world building is extremely bad. It starts with a battle, which reminds me of some mobile games (or, wait, this is based on a game "Alchemist Code", so, I guess, no surprise there), where you get top heroes for tutorial, but then something happens that they die, get locked up, lose their power or whatever.
During the battle you can see that "magic" (alchemy) seems to have no rules, because there are no actual patterns in the attacks or some common elements between the characters, that you would see if the rules are well-thought and "grounded". Same is true for equipment: there is a bike and a gun, robots, but most use swords, which seems unlikely unless it's something close to what we have in FFXIV (for example).
There are "mages" there, but it looks like they are not "mages" in our sense, more like regular mobs. They are said to be different from some other mages, but no details are shared on regards to how they differ.
Regardless of lack of explanations in a fast paced scene (which can be ok), there are hints that this world could be interesting, and you hope to learn about it, but then... You are transferred into modern world. Boring and uninteresting and slow.
The change of pace is too extreme. It's not a change of pace, that you can see in AoT or something, where you have a battle scene and after it's ended, there a moment of piece, where characters gather their thoughts and stuff. It's just... Too sudden and does not fit. If modern day started with MC (or someone else) rushing to school (for example) but then reaching it and getting into a regular-day pace - that would be so much better. Or use the the moment the girl gets isekaied as a transition and show her boring life as her flashback or something. Especially since she was supposed yo be "judged"
Yes, they then started trying to answer some of the questions but that felt half-assed and I already lost my motivation to continue watching it at about 35-40 minutes.
It's just that it should be a TV series with, potentially, a bit more linear narrative. Like 2 first episodes depict events before and during Destruk ending at his appearance. Then 1 episode that shows that girl in her daily life, how she starts hearing voices (maybe seeing things as well) with cuts from the "fantasy" world, she gets summoned in the end of the episode. Then in next one, she is introduced to current world and to characters, to their current struggles. In next one - flashback to what happened when Destruk appeared and how alchemy was lost and from the next episode onward - the actual "journey".
If done right the above can establish the irregular yet captivating flow that Deca-Dence had. And it could be a great anime. But as is - meh.
