After fracturing, subfracturing, and chipping away a few bits, I can't make use of all the groups generated in the process to recover all outside faces. Even if I try to combine groups and subtract/add some, I can't get a clean group of all the polygons on the outside surface.

Is there a quick vex way of doing this? Or another combination of nodes?

Cheers

Have you tried grouping the original geometry pre-fracture and Group Transfer?

Yes, I had done this but it didn't work perfectly

If you mean all the original outsite faces, this might work. Use a primitive wrangle with this:

it for some reason doesn't work. I think because my geometry and grouping have become such a big mess that I can't make sense of it anymore. The geometry and grouping was perfectly clean up until the moment where I blast some small chunks off. The new outside faces revealed by this blasting are the faces I can only partially bring back to this new outside group. But some of them I can't, or otherwise, it groups some faces inside.

Thanks everyone for the help, at this point, I gave up it wasn't that essential to what I was doing
I either messed up somewhere in my workflow the way I organised my groups, overwritten something I shouldn't have or soemthing else.

I was thinking of creating a sphere englobing my geometry, and with a intersect() or some similar function, grouping the faces whose rays would be first intersecting, but because the geometry is quite concave, I am not sure it would even work.

Cheers!