If you've got SCVs auto-repairing the Thor, then the Zerglings have to manually shoot them down before they can hope to achieve anything, and Siege Tanks kill Zerglings in droves until the next patch. Sure, if they're by themselves they'll get surrounded and killed by the Zerglings, but why would they be by themselves? For Terran, Marines, Hellions or Reapers can render Zerglings largely ineffective, and those are pretty cheap units to mass.



Marauders and Immortals are good against Ultralisks. Tanks less so, even though they're still good against pretty much all ground units. Ultralisks are actually pretty core in dealing with meching Terrans because they can absorb enough Tank fire to get in range, and once they do they slaughter them. Same goes with Marauders, to an extent, though it's much easier to kill Ultralisks with Marauders than it is to kill Marauders with Ultralisks.