Where are they going to trade?
Under the Commodity Futures Modernization Act, SSFs are supposed to be treated as securities at least as far as the application of the SEC's margin rules, etc.
A few SSFs trade on European futures exchanges I believe, but I haven't seen anything about where/if they'd trade in the US. Or whether it's likely there'll be any significant volume.
But if they trade on a futures exchange, you'd think they'd have to be traded in a futures account rather than an equity account.