Most games will counter this effect with area of effect damage. AoE has onagers for this purpose - which can easily deal immense damage to clumped and unaware archers - SC2 actually gives a lot of units AoE which is what prevents marine deathballs from being nearly as strong as they might otherwise be. 10 zealots and a clossus vs. twenty marines will be a pretty one-sided fight that will get more extreme the higher the numbers go.