Radar beams bounce off of a lot of things successfully, but attenuation becomes a factor. How much of the energy is absorbed? How much of the reflected wave is at the same amplitute? How much scatter effect do you experience from objects other than metal. How much of the beam just passed right through the object entirely.
We have to keep in mind that the primary principal of radar is reflection from metalic objects. We're not saying that is the onyl object that can reflect RF.
A lot of navigation
bouys and pilings have reflective tape on them to aid in successful beam reflection.
Metal is best. As a navigator, I would want to see everything. That's why AIS
is so cool.