That serbu rifle apparently works just fine nearly all of the time. What's concerning is the main failure point is directly towards your face. One thing fails (those breech threads) and that's it. There is no failsafe. And as stated above by fox, the failure point is not much higher than the design standard if what Scott said in the video is correct. Failures may be uncommon. Maybe this is the only one. (Serbu himself said it's the only one out of 14,000 guns). But that's really not great odds IMO.

I've witnessed guns fail because of overpressure. Here's a picture of a buddy's bolt from one such instance that I witnessed. Overpressure was enough to sheer the lugs right off the bolt. But the bolt didn't come out the back of the gun. Explosive gases have somewhere else to go instead of only sending things straight back.

Yes, it appears the ammo was the problem. And even Scott at Kentucky Ballistics says it wasn't the gun. But I'd be real curious to see if he ever shoots that rifle model again. I wouldn't. And I bet most others won't either. I predict that Serbu model doesn't have long after this.