I'm looking for a reason NOT to use premade fiberglass shells.
Any insights? Am I missing something?

If money is no issue, go with concrete.

Typically, fiberglass shells are much cheaper... at least in my area.

Concrete is twice as strong and twice as thick, hence the reason house slabs are made of it and not fiberglass. You also have all variables with concrete including shape, depth and surface finish. We build concrete pools, spas and swim spas for around the same price as fiberglass ones, so price if you shop around should be much of a issue ither.