I think there's a baseline of "keeping to the spirit of both characters" that everyone must follow for any of this subreddit to function. So in terms of who is writing the story, there's an expectation that the writer is respectful of both characters.

Basically if Tsugumi Ohba is writing this crossover between Light Yagami and Saitama, even though Ohba would say that the Death Note is absolute, we expect him to not be an arse or overshoot the spirit of the characters. So (assuming Ohba is a decent person and not a dingus) he would probably come up with an entertaining reason why the Death Note didn't work on Saitama.

The same can be said for ONE. Though he would probably make it a bit more comedic.