If the mini's reliability was even as good as the AR, most PD's and entry teams wouldn't have spent twice as much for ARs. AR's are the preferred choice because they are much more reliable in real life than most of the common public knows. What the heck, a lot of them can't even find Afghanistan on a world map, and think the AK has more range and power.
As for ammo, a lot of people confuse the comparison. Sure, .308 surplus is cheap - but it's not a hunting load. It's also out of date for military use, and not guaranteed any better than 2MOA regardless of the gun it's used in. Compared to any commercial hunting loads, sure it's cheap - and inaccurate. It's often illegal to hunt with, too, as many states have prohibitions against full metal jacket. And carrying a .308 on a long hunting day in the field - no comparison. Few hunters choose 12 pound rifles, they compromise and accept more recoil from 7 pound ones for the few shots they get. With 6.8, you don't even suffer that.
Aside from SSA tac loads, most available 6.8 ammo is loaded to SAMMI specs, as the ammo makers are more conservative than the gun makers. Their worst case is the SAMMI chamber, they don't want the liability which is a political/legal situation, and strangely backwards in this case. SPCII is safer.
Unless you can and will shoot SSA hot loads, or handload your own, most 6.8 ammo won't take advantage of the lower pressures with more powder. It all shoots slower than it could go. It will take a new designation to get the ammo makers up to the SPCII standard, like 6.8 x 43 NATO or something. Until then, it's marked as a hot load - and most aren't.
It's why Ruger is offering their AR in 6.8, at least you will get more reliability, and it will shoot almost every ammo on the market more accurately. The Mini? Just a niche rifle for those who like the M14 but don't want what? - the bulk or weight of .308.