The clamping elements are pretty snug in the 20mm holes. I sincererely doubt you could use them in a 3/4" hole without opening up the hole a bit, or sanding down the clamping element post.
They're up to the task of doing a panel glue up: e.g. cutting boards, the panel for a door, etc. I would not rely solely on them. They will provide a good holding action while you apply cauls, and more clamps (unless you have a bunch of clamping elements).
In terms of gluing up of thicker stock (e.g. leg glue ups, laminations for large turnings, etc) you are better off gluing in a horizontal position and using clamps for putting pressure towards the workbench, rather than laterally as the clamping elements do. But, that's not to say it wouldn't work-- you could use the clamping elements supplemented by more regular clamps to provide distributed pressure.