Here’s the thing. Peer review is not perfect. It’s not a panacea. It’s simply the basic level of due diligence. By submitting work for peer review, a scientist has allowed people outside her own team to critique her work. And the journal might require some changes to the paper based on the critique — anything from edits for clarity to requesting that the scientist perform another experiment in a different way. If a paper hasn’t gone through peer review, you should be more skeptical of it. Avoiding peer review means that the researcher decided to show the public her results before allowing those results to be critiqued by independent experts.
But, at the same time, just because something has gone through peer review doesn’t mean it’s been certified to be accurate. It just means that roughly three other experts have looked at the paper before publication. There’s still a lot of room for things to go wrong. Peer review is like the bouncer at the door. The bouncer doesn’t guarantee that every person in the bar would be a good person for you to date. Even if a paper gets through, you still have to think about it critically and evaluate it on its own merits.