Yes they could deliver huge loads only if they got there in one piece, which did not happen too much when they tried to supply Africa. I am not up as much as I would like to be on eastern air action but towing a huge cargo glider at approx. 140kts. would be an easy target for amost anybody without alot of escort fighters.
The annerversery of the Palm Sunday Massacure is coming up soon where B-25s and P-40s shot down huge numbers of German transports trying to deliver fuel to Africa. I do know on the eastern front they were so desperate for transports they used the He-177 as a transport instead of a bomber. It failed as a transport also from what I read.
If you were in the German airforce during WWII and got assigned to crew a Me-321 I don't think you would be as fond of them as you are now..... YOu would be too busy being dead.
But you are correct in that no one else in the world could deliver those loads.
I wish I could find a web site that had the loss rate for the Me-321, I am sure it would be very grim reading.
I know you like the aircraft Chrome and I don't mean to be so down on the plane but again if you were assigned to crew it you would have a different view.