Yes fiberglass can crack, but for gauntlets its a suitable material. Fiberglass is great for the kind of stuff we do, better and stronger than vaccume formed styrene and other plastics but by no means is it indestructible, it will crack under pressure.
I personally would rather have fiberglass armor over vaccum formed armor. Yes thats how the move armor was done but for trooping and what not the fiberglass would probable last longer under the wear and tear