While the initial results of the Peterborough SIB did not trigger payments to the investors, the many factors driving this first SIB's success or failure are still up in the air. This report from NPQ outlines the various positions and opinions. Perhaps most telling is that the UK government has decided not to continue the program as a SIB but rather to revert to a more standard pay for performance contract without outside investors.
I think the role/need for investors in SIB structures remains unclear.
First Social Impact Bond Fails to Meet Halfway Mark Performance Target - NPQ - Nonprofit Quarterly