There are issues however - mobile operating systems (especially iOS) kill background processes aggressively - and the user has very little control over this. Until that changes, you can have federated servers running on phones which will go offline frequently and will have to sync when they get back online. This is fine for some applications but perhaps not fine for others. The server also needs to be lightweight because of memory/bandwidth/battery constraints on mobile devices.
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There are issues however - mobile operating systems (especially iOS) kill background processes aggressively - and the user has very little control over this. Until that changes, you can have federated servers running on phones which will go offline frequently and will have to sync when they get back online. This is fine for some applications but perhaps not fine for others. The server also needs to be lightweight because of memory/bandwidth/battery constraints on mobile devices.