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

RickJ

If money is no issue, go with concrete.

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

spa and pool doc

Concrete is twice as strong and twice as thick, hence the reason house slabs are made out 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.