It's been a while so the details are not fresh in my mind, but it wasn't the easiest thing in the world. I think most of my trouble came from the general lack of polish on Kubernetes (from a cluster operator's perspective) than from the specifics of the Raspberry Pi. One thing I remember clearly is that kubeadm has completely failed to upgrade k8s from one minor version to the next every time I've tried it. I always end up just saving my k8s resources, blowing away the cluster, creating a new one, and resubmitting the resources to the new cluster.