The only downside is you can't really tell what the specific differences are between the materials. For example: I used the site to find an aluminum alloy for use in a group project. My fear is that there are properties about the alloy which aren't listed which will make it an unideal choice.
Professor: "Those are nice calculations you've spent the last 4 weeks working on. Unfortunately, the alloy you picked breaks down readily in the presence of monatomic oxygen, and is therefore an extremely poor choice for the skin of a spacecraft."