@Onsokumaru Announcer turning into a seraphim is deus ex machina no matter how and which way its being turned.. but it being deus ex machina isnt what gets people peeved off since well preshadowed deus ex machina is often used as event known as story or arc finale.. but its finale only because preshadowing is there.
Here is a video about how deus ex machina works either for or against the story. Its video done by Fandor and focuses on movies but same principle applies to cartoons and mangas aswell.
: https://www.youtube.com/watch?v=iviZR85VMcU
So what happened here in Tensei Shitara Ken Deshita?
So far existing foreshadowing for this event is random dream in the past with 1 panel of information but nothing to tie it into anything. 2nd is existance of an ability turned into a random narrating force inside protagonists head. Thats it.
Hell the master itself doesnt know wtf suddenly appeared out of a thin air because
he used random skill he obtained from dark knight 3 minutes prior which manifested voice inside his head into prettymuch all-knowing seraphim autopilot which took control of his body and teleported them away. Then died off
mangaka (and possible author himself) botched up preshadowing and thats what peeving people off. They want to like the story, they want to keep reading this without a care.. Im reading through web novel right now so i can give a coherent judgement for this because I dont hate the story.. but good god I hate deus ex machina.