Ceph CSI (rbd.csi.ceph.com) vs Ceph RBD (kubernetes.io/rbd)
I don't know the exact differences, but I was told from a Ceph CSI developer that the Ceph RBD (kubernetes.io/rbd) i.e. the in-tree driver will be deprecated in a few Kubernetes releases. And I don't have any references to any official documentation as this was a slack conversation.
So the CSI driver is the way forward and makes it more future proof.