I don't know about Grohe, but I have had a Hansgrohe handshower in metallic-appearing plastic for over 5 years now, and it still looks brand now. There are actually advantages to plastic: it doesn't burn your hand when you pick it up and doesn't hurt you or your shower if you drop it. I wouldn't worry about it.

Metal is heavy for handheld operations as well. When you hook the head onto the holder, if it is too heavy it will flop over.

Most of the handhelds I've seen are plastic, I think there are more advantages than disadvantages.

our handsgrohe handhelds are plastic and I thought the same. It has held up well in a guest bath. I bought same brand in my current remodel because I loved the spray.