Actually sapping makes sence and works in the way it, theoretically, would have. One of the (small) drawbacks of sapping is the build points needed for a sap point, and they are generally within range of the towers so you do have some losses from tower fire.

it does seem a little unrealistick however,

the principle oif sapping is that you dig a tunnel under the wall, then light a really hot fire in the tunnel to collapse it and bring the wall above down with it. So how come the unit doing the sapping can stay underground while collapsing the tunnel? Surely they would get crushed?

Maybe the unit should have to exit the tunnel before it collapses, and does say 50% damage to the wall, the has to re-dig the tunnel from scratch to get the rest of the wall down.

Just my two pence