Steel can be ''overcoated with fiberglass'' for a lot of different reasons and using a lot of different techniques. To begin with all ''fiberglass'' is not the same. Similarly neither is all forms of steel. The best steels are relatively corrosion resistant. The best coatings, zinc rich
epoxy primer, bonds tenaciously to steel and provides a good base for the final
paint job.
Epoxy with glass cloth can improve the protective nature of the barrier coating.
On the other hand, polyester tends to bond poorly with steel and where ever voids occur crevice corrosion is likely to occur as well.
Fillers are often used to fair up an unfair steel hull and these are often coated with
epoxy/fiberglass sheathing to protect them from blistering.
Epoxy is an excellent moisture barrier and there by reduces the opportunity for electrolysis.
In the worst case you have all of the worst posible worlds. A few years back a detailed survey of boatyards was made. When viewed over a long period of time, steel was the highest maintenance material typically used in boat building. (According to this survey, even higher than wooden construction.) Add to that a polyester/ fiberglass sheathing you have the worst of all possible conditions.
Jeff