Jon, we have GRP sidewalls and can only find positive things about them.
They do not transmit the cold as readily as Aluminium, they do not dent at the slightest knock, if you are unfortunate enough to damage it, it is easily repairable without having to replace a whole panel.
Only guessing, if it was involved in an accident on the road, and turned over, then it would probably end up as a mass of fibres, but even that is preferable to sharp jagged metal flying about.