Plastic actually makes phones more resistant to drops thanks to better shock absorption than metal. On the N900, the plastic is kind of soft, with detachable parts, a brick form factor, bezels, and a soft resistive screen. All this probably made it a very decent stunt phone; I have not seen many pictures of N900 heavily damaged after drops.
I expect the Pro¹ to be a lot less drop resistant with his harder body, much larger screen area, and curved glass with no bezels. Not planning on trying.