I’m not going to claim that all SEO plugins are bad or even that the one’s that turn out poorly (in some cases) have bad intentions. In most cases, the problem falls on user error more so than the plugin itself.
If you’re considering any plugins that offer SEO features, be sure you CLEARLY understand how to use the plugin and configure each setting for optimal results. Installing and/or configuring an SEO plugin incorrectly can cause great damage to your site or blog.
Circumstances where you should triple-check EVERYTHING to ensure you are not going to screw yourself:
Plugins that provide any type of redirection – 404 errors, 301 redirects, 302 redirects, affiliate link masking, etc.
Plugins that modify your Titles and Meta Tags.
Plugins that modify your page structure and/or Permalinks.
Plugins that modify other Meta information (robots.txt, unnecessary Meta tags, etc.).