As a Linux user that have just switched to KVM after a couple of years of using almost exclusively VirtualBox, I must say that VirtualBox is obviously better documented and much more user-friendly, at least for a beginner. To me and probably to many others it really was the gateway to virtualization. All things considered to me the main advantage of VirtualBox remains the easiness of setting up shared folders, something that I find much less practical on KVM (file permissions is a real pain with 9p virtio).
Except from that, virt-manager allows for a lot of customization and is not that difficult once you get used to it. From a philosophical point of view (and even security wise), it is clear that we should promote the use of KVM over VirtualBox (free software). However we want to make sure to provide the best out-of-the box experience otherwise we risk to scare new users away, which is much worse than using VirtualBox