What are the conceptual/theoretical and actual challenges to the realization of an ‘Islamic state’ as forwarded by March and Hallaq?

What are the conceptual/theoretical and actual challenges to the realization of an ‘Islamic state’ as forwarded by March and Hallaq? On the other hand, why does Salman Sayyid argue for centrality of the Caliphate? Is there a way to reconcile these competing discourses?