For an isekai, the concept is interesting, a hero who can't do anything but to use a shield?

Personally, I felt like it falls apart, however. Here you have a protagonist that is innocent and a common civilian, transported to a world he knows nothing of, and after the events of the second episode he radically changes from day to night, now bitter and resentful, throwing his personality out of the window and adopting a new one. If there had been a time skip between his arrival, his betrayal and its consequences, I'd certainly forgive this, but there isn't (or at least a relevant period of time).

There are many plot holes and plot-established rules that are constantly broken and then reinforced again, that goes from how the hourglasses and its countdowns work, what are the Waves the heroes must fight, the memory of filolials, why the protagonist was the only hero who had never experienced an RPG game when all the other heroes had (this may seem like a nitpick at first, but it kinda is the exception to the established rule), and so on. I feel like the story focused so much on making the viewers hate certain characters due to amount of trouble the protagonist has to go through that they forgot to tackle with the actual explanations of why they are in this world and what is going on.

By the time the whole conspiracy involving the Church and the Royalty is finally over, there were only 4 or 5 episodes left and they had to rush this explanation to end on a cliffhanger (pretty much expected). The writing also is plagued by amateur mistakes such as pausing an event beyond the character's control or a duel/fight/battle to discuss backstories or wash the laundry of all the stuff they didn't bother to show in the past episodes, all while the villains have to pause and watch the debate as if they were stuck in an unskippable cutscene in a videogame rather than strike them down; not to mention the pathetic morality and lesson-giving that everyone's pretty much tired of already: "We can't kill people, no matter how bad they are".

You have rapists, torturers, slavers, murderers, fanatical leaders and politicians ready to murder or destroy the lives of anyone as long as they gain from it (even genocide!), but of course, you can't kill them! You just leave them alive, living in their former positions of power just expecting they won't do all of it again! You'll know this when a certain "Bitch" and a "Trash" gets to stay in their party and living in a castle, respectively, so they can do more bad stuff that wouldn't happen if the protagonist had some balls to end their miserable existence.

Another issue is the soundtrack that is one of the worst I've heard (sax on fight scenes? loop of nails against a chalkboard?), which may have to do with the show's editing rather than the actual soundtrack, and the weird use of CGI in some episodes (you'll know when you experience it as I think I've been spouting a lot of negatives).

Raphtalia steals the show, pretty much. She's cute, interesting and kicks ass when she has to, but don't expect romance. Filo is your standard cute kid and a somewhat daughter figure for the protagonist.

Overall, I haven't read the manga, so I'm assuming it is better than the show, me and my friends were almost dropping it by the end of the Conspiracy "arc" and realizing there were still four episodes left, so I guess (depending on the person) a person watching it alone would've given up much earlier. Could've been much better if they dedicated some more time on the writing.