ling infestor also works really well against Terran Mech although the transition should be into brood lords rather than ultras as Ultras aren't really worth their investment in my opinion.
I think most terrans that spot Infestor Ling will go a more heavy Tank/Blueflame comp over a heavy Thor comp, and that will pretty much negate your army.
This is for mech btw. vs Marine/Tank, it depends wether you can get fungals hitting marines imo.
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