Yes, there are plenty of downsides. Fiberglass and resin by the gallon is expensive and time consuming, not to mention heavy. It's also very hard to measure internal volume precisely on something that has a really odd shape like that, so ported fiberglass stuff is pretty rare, most are sealed. Ontop of that, the finishing process is a P.I.T.A as well. Tons of bondo, primering, sanding, more primer, paint, clearcoat, more wetsanding and buffing.
Upsides? They look cool, thats about it.
The only other really good use is if you use fiberglass to make the bottom of your enclosure in the trunk, as it can get the most volume out of a spare-tire wheel well. But that's all I can think of off hand.