Note: just realise I’m speaking in respect to the anime and not the visual novel.
I think, in a way, “key magic” made some of the anime more realistic - ironic considering it involves magic. The main example is Little Busters.
Little Busters Refrain (anime) plot twist spoiler
Convinently every girl Riki talked to had some sort of regret or tragedy in their lives that need to be resolved. Thanks to “key magic” this is explained. With Kyousuke, Kengo and Masato making that phantom world with the 5 girls,thus, Riki was kinda pushed into talking to the 5 girls, rather than it being a convinent coinicidence. Do I think that is “Deus ex Machina”? No way, it actually gave the plot room to have more depth to it. The literal disappearance of Mio Natsumi was a fantastic tool to express the idea of loss of identity, and envoked a larger emotional response for me. The repetition of June 20th for Kurugaya also made her story more powerful for me because the repetition emphasised her desire for stay with everyone. Not to mention that overall, the “phantom world” was a fantastic plot twist and made the ending immensly entertaining
However, for Clannad, it did kinda feel like a “Deus ex Machina” in a way
Clannad after story (anime)major spoiler
The magic of Clannad was always referenced throughout the anime so it wasn’t put in there out of laziness or to convinently wrap up the story. However, when Tomoya went from Ushio dying to Nagisa back alive, it felt like a “Deus ex Machina” method to make the story a happy ending. I’m not saying it’s bad thing, I feel like this method completed the story because after Tomoya’s large ‘self discovering’ or ‘self monologue’ (however you’d like to describe Tomoya’s scene after Ushio dies), they couldn’t leave it with just Ushio dead, or if they did, I’d probably be pissed with the story. But, at the same time, it felt like the producers wanted to go down the “bad route” to make clannad so emotionally strong and give the story depth , but then revert back to the “happy route” to make everyone happy, and thus, get views and sales. What do you reckon?