Mag changes are difficult under the best of circumstances. Fortunately, I get to put rounds through a lot of the new weapons. The 6.8 SPC is really impressive. I was surprised.
I have a T/C barrel for my G2 carbine in 6.8. Better trajectory/ballistics than the 30-30. I use it for deer.
You are comparing it to the wrong sort of tool. It's designed for those, who for one reason or another, can't easily carry a full sized weapon, nor even a "compact" one like the CAR, in whatever caliber.
It's not designed to be used at 200 yards, and rarely at 100 yards. It's really designed for the same mission as the M-1 Carbine was originally intended for, vehicle crews, artilleryman, and behind the lines support personnel. For that it's more than OK, when compared to the 9mm pistol it mostly replaces.