OK... Pros are that the shape is there and you don't have to sculpt one. You can modify the shape with foam slivers before you fur it up. You have fixed attachment points which can come in handy for things like ventilation fans or lights with their battery packs. Moving jaws are easier to attach as is the nose and teeth. They are usually cooler as you don't have all that foam to contend with.
Down side. They can break if stressed by weight or even through material fatigue. They are hard to repair if you don't know what you are doing. Fiberglass is fickle, strength wise. The caster of the head has to get the mix right. I used to work in composites making military aircraft parts, so I know. They are normally a lot heavier than foam heads. You have less freedom of design as you are limited by what is available.