If you think about it, its no different to banner towing or tugging a glider
I can't agree. A banner, or glider on tow, is much more acting as drag to the aircraft, than directly as weight. The nearly pure weight force (as opposed to drag) of the concrete hanging from the tail would very certainly act to put the aircraft way out of C of G limits, and make it very dangerous, should it have been allowed to stall.
The determination of this is based upon the angle of the "tow" rope. Down = weight, aft = drag. Obviously, both cases have elements of both forces, but clearly the weight is evident as the primary force with the concrete.
I'd say that pilot was
very lucky to get back to the ground safely.