How risky is it to recieve "money under the table"

Private English teaching, English teaching cafes, and other services that students in Japan usually find jobs in I've notice often pay under the table.

I'm quite curious, is there any true risk to the students taking jobs like this legally? I know some companies in the past have gotten in trouble for hiring workers without proper paperwork, and even my previous study abroad college department actually had me working as an English teacher in the international center without the paperwork completed (mostly because they simply never checked, or even asked, nor did I know about the situation)

Who bares the risk here, and how high is it?

I can't actually answer your question, but I teach a lot of private lessons that pay under the table, in fact, most of my income comes from under the table private lessons.. I've read somewhere that the Japanese gov't won't really look into anything under ¥500 000 per month, but don't qoute me on that.