Note that both types are just a skin laid over a frame and backer of some sort. Fiberglass sheet is less vulnerable to tears than TPO or rubber (EPDM), but the construction is otherwise largely identical. The edge seams are sealed with lap sealant (aka caulk), as are joints where the many things that stick through the roof penetrate it.
I think both fiberglass and TPO sheeting makes an excellent roof material.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC