Righthaven didn't own the copyrights on the material; they were simply "granted" the right to sue, which turned out to be bogus from a legal perspective. To enjoy the benefits of SFC enforcement, you must assign your copyrights to SFC thereby avoiding the Righthaven situation altogether.

Looking at SFC's website, it appears that people actually assign their copyrights to SFC.

I can see why other projects are hesitant to join in.

not exactly.... they are enforcing their own copyright, and by virtue of the license wording, using it as easy leverage to insist the infringing product is cleanly complying with similar licenses before granting a new license.