The chests is correct but some especially elvish and magical seem very rare - need more options to win then like noble and holy artifacts in the arena - you can't progress the command centre without them - I got a couple of stolen articts in the 1st battle, just lucky i guess, good hunting.
Thanks for the info, I have discovered this from a recent update, and had pleny of artifacts (esp. noble artifacts) to swap out. I now have my forces up to level 10 at the Command Centre, which I believe is the current maximum. It also helped for magic artifacts that some Squad leaders needed to max out, so cheers for that again.