Each statistical computation benchmark is a specific posterior distribution with validated posterior expectation values not a generic model.
SBC tests the consistency of a computational method of an entire ensemble of posterior distributions generated from prior predictive data which (usually, although not always) requires that the computational method fits all of those posteriors well. If the priors are wide then more and more posterior behaviors will have to be accommodated which may frustrate the given computational method even if some of the posteriors distributions fit fine.