Everyone used to prototype in composite. Whether it was all fiberglass, kevlar/aramid, or with carbon depended on the budget of the particular design process as well as if they thought they could sell a couple. I know a local couple around here who still paddle kevlar/glassfiber Perception Pirouettes.
Prototyping in plastic is less expensive and the manufacturers can get the boats out to their pros and let people beat on them. That's why most prototyping went that direction. Every now and then a boat still ends up in composite for some odd reason. Usually, it's an indicator that it won't be mass produced in plastic. The metal molds for plastic boats is expensive. A mold for composite boats is less expensive so (if the perception is that the market is there) then a company can sell a few composite boats without tooling up to go to plastic.
Composite boats are very nice. Stiff. Light. But, unless you are paddling deep spots (and know how to avoid rocks) then you are making very many repairs.
Carbon CAN be fixed, but it's never quite the same.
Kevlar is more resiient in the long run, but very flexy without foam core or some carbon mixed in for stiffness.
Fiberglass is in many forms, so you need to know which variant of glass is used.
Composite boats are nice, but you really can't rock spin, splat, play in shallow features, or do any low volume creeking in them. It's best reserved for race boats, or class III/IV cruisers where the object is to have nice lines while avoiding rocks. If you get a playboat in composite, chances are it will look like Frankenstein's monster by the end of it's first season (with all the repairs) and probably be much heavier than when it started (all the added weight of the repairs). But, it stil is very fun to paddle a composite boat.