internal balancing

I would like to know why it is detrimental to the engine if you would balance the pistons and conrods after the crankshaft is balanced on its own? if the pistons and conrods all have the same weight and the big ends of the conrods are balanced all the same, would this not mean that the crank shaft assembly is still balanced?

Yes and no.

The counterweights, if used, are there to counteract the masses of the piston and connecting rod, to help reduce the loads on the crank and bearings.

With minor changes in piston/connecting rod weight it isn't too much of a problem if it's a few grams different from the 'ideal' counterweight mass, but when there is a significant difference it can affect the crankshaft and bearing loads.

That said, I'm not totally convinced about balancing to very tight tolerances, as variations in combustion between cylinders will also affect the loadings.