Someone who listens to Shaykh Hamza Yusuf explained it to me as follows. Shaykh Hamza believes that the Sufi tariqas have in the past been sources of disunity among the Ummah. He also thinks that what the Ummah needs right now more than ever is unity. Hence, he discourages people from joining a tariqa. I see the reasoning, but on the other side is the fact that for many people, taking baya in a tariqa is an extremely powerful means to self-recitifcation. I don't think people should be discouraged from availing themselves of this spiritual medicine.