Such blatant disregard of civility, to everyone and anyone, should have had Sombra dealt with long ago, but his power truly is overwhelming. He is an master of armed combat and a dark magic not well understood by anyone but perhaps himself. He is terror incarnate on the battlefield, sowing fear and destructing in equal measure. He has historically fought Celestia to a stalemate, though his callous disregard for collateral damage has forced Celestia to lose the advantage in every encounter.
As a war leader Sombra displays competent, if basic, tactical skill, favoring aggression above all else, and preferring to destroy objectives rather than take them. He has legions of mentally dominated soldiers at his command, and also employs powerful “crystal golems” of unknown make as shock troops; a combined force that is difficult to counter at best.
Sombra’s unfocused strategy of apparently attacking everyone at once has fortunately worked against him–were he to focus his efforts on one country, he would doubtless overwhelm them. But as it is, he held within the Crystal Empire, fighting numerous wars all along the borders.