science vessel fires emp (also irradiation) in the second round of the battle and only once during the entire battle.....it would make much more sense for it to fire at the start of the battle because by second round our own battle fleet takes down most of the shields....as a support unit it doesn't help much if it fires too late and only once

P.S. also toss have shield restoration ability and xerj have armor regeneration....while terran doesn't have any special ability...only useful unit against them is science vessel