Lacking the money to buy each of the products I'm going to review, my approach has been to a) look at the official product specs and b) do a thorough read-through of all the customer comments on the item to get a sense of the effectiveness of the item and any potential design problems my readers would want to know about. To be honest, I think this is probably a better way to review products anyway, as a lot of quality issues don't become apparent until after months of continuous use.
Right now, my plan isn't to limit my reviews to the best of the best in my niche. Ideally, I want to cover a range of products of different quality levels, so my readers can see I'm not just in this for the money. I'm also hoping it will let me snag people googling for inferior products and let me point them to something that would better serve their needs.
But here's the issue. I was explaining the plan to a couple of friends and their immediate concern was liability -- not from readers buying a product and not liking it, but from manufacturers unhappy about design flaws and reliability issues my review has pointed out.
Like I say, I'm basing most of my work on Amazon customer reviews -- they're hardly peer-reviewed studies and, while I'm careful to only include trends in my reviews (i.e., if a bunch of people say their product failed after less than a year, I'll mention it, but if one person says it spontaneously caught fire and burned their house down I probably wouldn't) they can be faked. I was considering linking to individual reviews as "sources", but that's hardly evidence of a trend.
So, yeah. I want to be forthright about problems with the products I'm reviewing, but I don't want the manufacturers to come after me because I'm saying critical things about their products that I can't prove.
I know the rule is, "Don't take legal advice from the internet" (and, if it matters, I'm in Canada, though I'm using Amazon.com's affiliate program right now), but how do experienced Amazon Affiliate reviewers handle this problem? Do you take any precautions to limit your liability? Have any of you ever had manufacturers come after you for a bad (or at least critical) review?