To add a bit more to this (excellent!) response: kernel modules like the VirtualBox extensions must be rebuilt on every kernel release. There are a couple of mechanisms in place to automate this, and most notably in Fedora land, we have DKMS and akmods. akmods is what most of the third party drivers in RPMFusion use, and rpm-ostree does have mechanisms to be able to install these. However, the VBox extensions use DKMS, which rpm-ostree does not support.