See this thread for more info:
http://www.telerik.com/forums/alternaterowbackground-doesn't-merged-with-rowstyle

It's  really common scenario, because a standard WPF DataGrid works like this.

А new virtualization mechanism for ColumnGroups will be introduced with Q2 2015. A property EnableColumnGroupsVirtualization is exposed with default value True for enabled virtualization. Setting it to False will enable the old logic.

It's so strange that Telerik's components don't support such basic mechanics of WPF...

The same works fine with the old Nested mode.

RadGridView's FooterRow is still part of the visual tree, even though its Visibility is set Collapsed. 

Available in LIB version 2016.3.1121, it will be also available in the 2017 R1 Release.

Enhance aggregate functions (like Sum) in footers so they support expression columns in RadGridView. 

See the attached project. It works for DataColumn, but raises an exception that no suitable generic Sum method could be found when I set the Sum footer for an expression column.

It would be very nice to have a "transpose" functionality in RadGridView. There are ways to transpose data instead of GridView, but this way it would be difficult to configure bindings, styles and other parameters. PivotGrid isn't suitable in this scenario because of lack of editing.

There is an example of what is needed: https://rotatedwpfdatagrid.codeplex.com/. I was successfully using this example with a standard WPF DataGrid, but I don't know how to accomplish this in RadGridView.

There are already a few threads about this feature:
http://www.telerik.com/forums/transposing-columns-and-rows
http://www.telerik.com/forums/radgridview-can-be-transposed
http://www.telerik.com/forums/displaying-grid-data-in-column-row-format
http://www.telerik.com/forums/reverse-column-and-row-in-the-grid

Thank you!

Fixed for GroupRenderMode="Flat".