Fiberglass is not a particularly common material for garage doors, though they do have their adherents. The biggest advantage of fiberglass is weight. They are typically framed with aluminum and can come in any number of colors. Fiberglass garage doors are among the favorites of people who live near the ocean because they are resistant to corrosion.
Fiberglass doors are not the best solution for every situation. If security is a major concern, fiberglass should not be your first choice. Additionally, fiberglass doors are not good insulators, though they can be fitted with insulation. That said, they are not recommended for cold weather because they will not hold up well.
Fiberglass garage doors deteriorate over time like anything else. Often they will develop cracks which will not be fixable. You can at least be reassured that they will not dent.
In terms of maintenance, acrylic paint is recommended because it’s durable and easy to work with. Simply washing the door down the panels well. Sanding is not recommended because fiberglass scratches easily, and could even be a health hazard if the powder becomes airborne. Apply a primer first and then the acrylic.