The interface IApprovalDefinitionRepository has some basic methods I can mock against but if I want to unit test the _approvalDefinitionRepository.GetAsync(ContentReference), that I guess many will use, it's extremely more work to be done to mock.

Before reaching the interface and method  IApprovalDefinitionRepository.GetItemsAsync(IEnumerable()) that is unit test friendly we go through several static classes/methods (at least one is even internal). It's not that easy to unit test.

Please make it more unit test friendly so that instead of looking at Uris in the IApprovalDefinitionRepository.GetItemsAsync(IEnumerable()) I can match against ContentReferences in _approvalDefinitionRepository.GetAsync(ContentReference).

Example

_approvalDefinitionRepository.Setup(x => x.GetItemsAsync(It.IsAny>()))
.ReturnsAsync((IEnumerable inUris) =>
{
if(inUris.ElementAt(0).ToString().EndsWith(myContentReference.Id.ToString()))
{
// Do stuff
}
else
{
// Do stuff
}

return new List { new ContentApprovalDefinition() };
});

would be more easy if I could just do 
_approvalDefinitionRepository.Setup(x => x.GetAsync(myContentReference)).ReturnAsync(...);