There's a very big distinction between saying "I consider X harmful" and "X [is] considered harmful". In the former, it's very clear that you're expressing your own opinion. In the latter, it's implied that your opinion is "the" opinion, and that X is considered harmful by people in general, not just by you.

(At least, that's how I've always read it. I suppose one could argue that "X considered harmful" actually should be read "X [is] considered harmful [by me]" or maybe "X [should be] considered harmful", but I don't think either of those interpretations are obvious.)