The early Yokomo YZ-10 had a magnesium front bulkhead. It was too weak/brittle and, when it became available, most people replaced it with the RPM plastic/nylon front bulkhead. Magnesium is just too weak and too pricey. When cars are already bending/breaking good aluminum alloy parts, 'almost as durable' isn't good enough, especially for a very small weight savings. If you even save any weight. You'd have to make the magnesium part a lot bulkier to have a chance of it holding up and not breaking.
Serpent SRX8 electric
Serpent Cobra 811TE-E
Serpent S120LTR 17.5 1/12th and S100LTR Pro-10
Serpent S411 Eryx 4.0 USGT
Team Serpent America and Desoto Racing