I know this is old but somehow I missed it until now and it probably deserves some discussion.
I have also heard both. As I understood it the hardness allows for it to perform as 3 while also stopping 5.56 threats. However that hardness also makes it very brittle if attempting to form it.
If it's too brittle to form it makes me wonder how well it would actually perform against higher velocity threats like 5.56. I've been looking for some solid info to back up either claim but no success yet
I'm inclined to trust Tencate's testing, as opposed to random joebob shooting stuff on Youtube. That being said, I do not see any of their alloy plates appearing on the NIJ's CPL list. Take that as you will.
Since these AR680 plates are a form of steel, all the OTHER issues of steel still come into play-spall, weight, etc. I've not seen any testing on these to show how they spall versus an AR500 plate.