There is only one reason anyone would ever consider purchasing an underground bunker. That reason is SAFETY, so why then would you purchase an inferior product that is prone to failure? While fiberglass products may be good solutions for cars, boats …etc, fiberglass shelters are risky ideas with a proven rate of failure.
Fiberglass products are built from pre-designed molds that are costly to manufacture so having a custom design or to make changes to fiberglass shelter is next to impossible unless you are willing to pay tens of thousands of dollars to have a custom mold built that will likely set you back months from your desired completion date.
Fiberglass shelters are only a shell with no interiors. These interiors are built using wood products which over time will mold and rot. Another downfall to using wood in your underground shelter is the risk of fire and with the fiberglass itself being highly flammable making it the last place you would want to be during a fire.
Fiberglass products have a gel coating that is designed to protect them but when buried underground these coatings tend to get spider cracks which then allow the earths moisture to start attacking the structures themselves. When an underground shelter is built from steel it will expand and contract with the earth’s movement but fiberglass lacks these characteristics and begins to breakdown over time leading to premature failure.
The New Hampshire Materials Laboratory published a study in 2006 in regards to the leading causes of fiberglass structure failures. In this article http://nhml.com/hydrolysis-of-fiberglass.cfm they name the leading cause as Hydrolysis (delamination, blistering & bleeding). In this article they explain how many times they are asked to perform a failure analysis on something built from fiberglass. They also go into detail to explain the chemistry behind these failures and if you are seriously considering purchasing a fiberglass bunker then you should definitely take the time to read this article.
A quick Google search will lead you to the many failures surrounding fiberglass shelters resulting in many lawsuits and broken promises so when you are choosing an underground bunker, bomb shelter or any other underground shelter then please keep in mind that steel will always be your best option.