I consider anything that causes malfunctions or voids warranties is "not good". Also besides CMMG, how about Panther Arms DPMS: DPMS: AR-15 Rifles, Parts and Accessories.
Visited with Tech support today (2-22-10) at Rock River Arms, will void your warranty if you use Wolf steel case or any other steel case ammunition in their rifles/upper.
Spoke also with Del-Ton Del-Ton, Inc. A complete line of AR-15 Rifles, AR 15 Parts and AR15 Accessories. and using any steel case ammo, including Wolf (which is an importer not a manufacturer) or reloaded will void the warranty.
Also includes Ruger, and probably more have issues with steel case ammunition.