I can't argue the point since I don't really know. Maybe I mispoke, but I THOUGHT they were proven. Haven't they been around for a while or is this a newer design? I do know that every Sig pistol I've owned has performed flawlessly.
The 556 is based on the SIG 550/551/552 rifles that have been around for 30+ years. You might say that is proven.
Or, you can just throw out a one liner without any factual basis.