One thing that it entails is that the kernels available in Debian 8 backports are not compatible with the VirtualBox guest tools for Debian 8 that are published

One thing that it entails is that the kernels available in Debian 8 backports are not compatible with the VirtualBox guest tools for Debian 8 that are published by Oracle. So one has to make the choice on Debian 8 between a kernel that supports version 2 control groups and having VirtualBox device drivers for things like the display.