These were printed on their sides so the layer lines wouldn't be an issue with the force that would be applied. I've been 3D printing for years and have definitely learned that the hard way from having many things break. The most common filament used is PLA which doesn't hold up to the heat and sun of summertime and would soften and deform the part.
I printed a belt clip for my used Uniden SDS100 scanner that i got years ago with PETG (has a higher melting point so it won't soften) and it's been holding up great. This clip was also printed with PETG.
Haven't run across a need for any carbon filament yet, but who knows what the future will bring.