anti-Necropolis faction

Absolutely. Goblins – shamans, in a decent quantity – significantly hinder a hero casting spells.

How exactly do they hinder? Does it cost more mana to cast a spell, does the spell weaken, or does it affect their own units? Please tell me, because I don't know.
P.S. I really don't know, so my message doesn't imply that you are wrong. :)