The BillboardTransform qml item doesnt work. The problem is, that the billboard transform is applied to the localTransformation matrix in a qdeclarativeitem3d.
The billboard transformation that is used just sets the upper 3x3 matrix to the identity. This is a problem, because the localTransformation matrix is afterwards multiplied to the modelview matrix in the drawTransformSetup. But multiplying the identity matrix is not changing anything in the modelview matrix then.
In order to work, the billboardtransform has to be applied to the modelview matrix directly.
The BillboardItem3D however is doing this correctly and works fine for me.