If ARM architecture is selected as target Syncfusion controls do not render in designer. As soon as I move the architecture to x85 or x64 controls render correctly. The same experience happens in Blend. I'm using Visual Studio 2015 Professional.
I have confirmed that once the control is deployed to an ARM based device the controls will render correctly. The device that I'm using is a Raspberry Pi with Windows IoT Core. That shouldn't matter in the designer though.