Hey, hold on. Never hired anyone and paid them under the table. The contract work my company does is employing temp labor. The risk involved is simple, you can either do it legit (or semi-legit) or go under the table. We've always done it by the book. If someone gets popped by the SSA, we'd just fire them. No big deal.
In years past it might have worked to kind of say their work authorization or SSC looked sort of legit, but not since E-Verify in 2010. Took a huge bite out of our labor pool. As a result turnover has skyrocketed and productivity has gone way down.