Arbitrage is possible from any traffic source. Anything is possible if you are highly skilled at media buying and tracking.
When people are making a great profit from a "hidden" or "unusual" traffic source, they want to keep this a secret for as long as possible before they start blogging about it or discussing it on forums.
The general rule of thumb. If you read about it in a case study, the high-profit days have already been achieved from that traffic source.