People like long stories spanning years over a characters lifetime showing their growth along with their allies, enemies, and world as a whole. It worked for Berserk, it worked for Kingdom, and it absolutely, positively, did not work even slightly for Grancrest Senki.
The issue with Grancrest Senki is that the writer forgot to put the story in the story. the best comparison to make is to a Minecraft content creator who "did a little mining off camera". In between each episode, and occasionally each scene, the show will lurch forward to give the viewer a recap of the awesome plot that happened off camera. The point at which this comparison breaks is that after your favorite youtuber does his off camera fuckery, he will take those resources and make a nice house or upgrade his gear, Grancrest Senki on the other hand simply tells you "Hey, look some cool stuff happened while you weren't looking, now time to catch up with these cardboard cutouts we like to call characters! oh would you look at that time's up see you in a few years!"