Octrees can be used for non-point geometries too. You just have to store some geometries at non-leaf nodes. When inserting, you search the tree for the deepest

Octrees can be used for non-point geometries too. You just have to store some geometries at non-leaf nodes. When inserting, you search the tree for the deepest voxel which encapsulates the entire geometry (or, equivalently, its bounding box). At least that's how I've always done it. The advantage of R-trees and kd-trees is that you can partition the space more intelligently. With octrees, a voxel is always divided into eight equal children, even if one of the eight children will contain all the points/geometries. It's usually not a big problem, but if you have a few small objects with lots of detail and far from the origin, you might get a poorly-balanced octree with some very deep branches. As you mention, R-trees and kd-trees have slow insertion, but if you're path tracing a static scene and the rendering is going to take hours, you don't really care about that.