Its not too good to be true, there are lots of products that do a good job on rust, even petroleum jelly from the local autoparts store. None of them replace lost metal or are a simple one-step process. People have had good luck with Ospho, Metal-Ready from POR-15, etc. They're all fine products, but all require removal of excess scale, application of product, thorough rising, and prep/paint afterwards. None are very useful inside chassis rails, between sandwiched panels or other places where rust is most serious.