I'm open to some tweaks to Mine Berserkers, but your suggestion feels off to me. They are blowing themselves up, that is by design an offensive ability. In addition to that, Uruk-Hai are good the way they are now and should be and feel more powerful than Gondor soldiers, so reducing their base stats is a no.
If we were to add something to Mine Berserkers, I would look at something like a temporary debuff or disabling target structures. I can also see moving them to level two.