In total fixture replacement, it is nearly impossible to transfer a one-piece bathtub/shower unit into the same spot. Manufacturers responded to this problem with nits that go in as two, three or four separate pieces. This creates a number of seams that have to be sealed and maintained.
Fiberglass is similar to porcelain and solid surface polymer in the look and feel of the surface, though paper-thin. That means it is not as strong, more likely to crack and more easily scratched. Over time, fiberglass units can become more difficult to clean.
Most fiberglass units come with a manufacturer warranty in the 1-5 year range, though the installation quality or seam durability will not be covered.