If you look around and know the differences, stainless fittings are available. Yes, more expensive, but in some locations make sense---well to me anyway.
Stainless firewall bulkhead fittings. Definitely steel or stainless adapters on the engine-although there are millions of GA planes flying with AN grade aluminum adapters. Notice I said AN grade aluminum, not imported aluminum versions available at online stores. There is a difference. I'll say that YES there are thousands of experimentals flying with imported adapters, but most are in a low stress area. But they are generally of a softer grade to save costs. Thats great on a 'race car/street car' thats on the ground. And---the suppliers/manufacturers dont know or wont tell you the grade.
Make good choices
Tom