The downside is alum bends and is not easily straightened back to true again. Some alloys of alum will break when you try to bend it back. Plastic has some give built in and will bend up to a certain point, plastic is also lighter.

If the chassis is like the FLM one for the Slash, it will let road debris in unless you build some kind of shields to stop the trash. I had this prob with my UE SM and Dace Pred and cut some thin lexan to use as shields.