That's a good point. A lot of bad reviews are clearly user stupidity: I even see ones where they obviously bought the wrong thing, had unrealistic expectations, etc., and then gave a poor review based on this. Or reviews where the merchant (usually Amazon) screwed up somehow, so they give a poor review to the product.
On the other hand, positive reviews are frequently either lies or fluff, so you're exactly right to weight them much less. A lot of them look like they're written by shills, some of them even clearly say they got the product for free or cheap in exchange for a review, and many are written shortly after acquiring the product so they really haven't had enough time to fairly test it and see if it's reliable long-term.
Also, well-written negative reviews will go into great detail explaining exactly why the product is crap, so look for those. I'm remembering a great (and long) review I read for an automatic cat litterbox which attaches to a toilet here, and sometimes ends up cooking cat turds...