I almost exclusively use the clamshells to pick up rakings. I sub out my stump grinding so it's rare for me to need to cleanup a stump, most homeowners leave the chips and others hire my sub contractor to cleanup.
I have cleaned up a few and I use them to move wood chips on my property, they do work great for that kind of work.
As for cleaning up rakings, when the clamshells do not work great, is when you are trying to scoop up larger debris. A 4-6" diameter piece of firewood can get stuck between the shells and not allow them to close, allowing all debris that is smaller than that to fall through the space between. As I mentioned above in your quoted text, I will grapple a load or two, getting that bigger debris before raking the smaller debris back into a neat pile for the clamshells.
With a small enough pile of debris it is easier and quicker to use a scoop shovel than to install the clamshells. If you have a big enough pile(s) for more that 2-3 clamshells then you'll be glad that you installed them.
As for loading the debris, typically they go in a dump truck but sometimes on a flatbed trailer depending on how full each is with other debris. In an empty dump truck loading from the back I could probably load 20 or so loads, never really had that senario to know for sure. But once you fill the back of the truck, set the clamshells on the floor of the bed and drive forward pushing debris to the front. An articulating loader, especially an extendable boom, should be able to load much more than my mini.
All in all, yes I think they are efficient and especially for woodchip hauling and stump cleanup they work much better with my mini than a bucket as I run a wheeled mini on bald tires, so I often don't have the traction to dig.
