Who faced to with the same limitations.
Error in PowerShell if only one item in XML (one report or one DataSource):
Publish-SSRSProject : The property 'Count' cannot be found on this object. Verify that the property exists
Error, if there are not DataSources in the rpproj file:
Publish-SSRSProject : The property 'DataSource' cannot be found on this object. Verify that the property exists.
You should add at least two items for every node:
<ItemGroup>
<Report Include="TestReport1.rdl" />
<Report Include="TestReport2.rdl" />
</ItemGroup>
<ItemGroup>
<DataSource Include="SharedDataSourceTest.rds" />
<DataSource Include="SharedDataSourceTest2.rds" />
</ItemGroup>
Via Visual Studio:

Error here:

(before merge #25)
Would be grate if anybody remove these limitations.