I've also heard that about low powered ammo - I.E. cheap ammo that many of us practice with. Also it would bother me that I couldn't buy a replacement bolt from multiple sources, KAC has a proprietary bolt design. This is one case where the saying "two is one and one is none" really could bite you!
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek