I've heard that the manga is different, I don't know how its different so this will be my critique on the anime alone.
Radiant had the potential to be something far greater, it even displayed that for a portion of the show, don't simply look at the score I or anyone else have given it and pass it off as bad.
Radiant is your basic shonen anime with your pointy haired snot-nosed brat protagonist who wants to save everyone and stop the bad guys. Nothing really unique there. A good chunk of the show is spent dabbling in miscellaneous adventures that don't add anything to the story, this is where most people dropped the show and why Radiant is rated so lowly. I would say Radiant doesn't actually get to be pretty good until the second arc and the climax of the second arc (which is basically a quarter of the show.) If Radiant had pushed out content as heart-racing and emotionally moving as the second arc did but for the entire show instead of wasting screen-time, then the show would have easily been an 8 or a 9. Radiant very much makes me think of Black Clover in this way, an extremely mediocre and unoriginal start of the show, the setting, and an unneeded time waster of an opening explaining the show to us every episode LOL. But with patience and time you'll see that there is something more to this show.
Unfortunately I can really only commend Radiant on a job well done for about 5 episodes but I still enjoyed this show and am looking forward to the season 2 since it was announced at the end of the last episode. I hope season 2 will continue with the level of content that the second arc ended with.