I agree that EY is probably overconfident in MWI, although I'm uniformed about QM so I can't say much with confidence. I don't think it's accurate to damn all of Less Wrong because of this. For example, this post questioning the sequence was voted up highly.

I don't think EY claims to have any original insights pointing to MWI. I think he's just claiming that the state of the evidence in physics is such that MWI is obviously correct, and this is evidence as to the irrationality of physicists. I'm not too sure about this myself.

As for why SI's approach is dangerous, I think Holden put it well in the most upvoted post on the site.

Well there have been responses to that point (here's one). I wish you'd be a bit more self-skeptical and actually engage with that (ongoing) debate instead of summarizing your view on it and dismissing LW because it largely disagrees with your view.