Currently goblin shamans dont attack or move from their spot unless they see someone in their field of view, or unless someone gets very close to them.
I fought goblin army of 24 goblins with TWO shamns but their field of view was blocked by trees so while other goblins were bravery dying on frontlines shamans passively stayed right on the spot until only 6 goblins lefted, and then they just retreated. The fight was very easy like this but only if shamans did their job it could be absolutely different.
I think their AI must be changed to be more agressive like that of overseer.
I think its more about general AI desisions for battle then for shamans (ranged advantage/or not).
While I stormed big goblin city (with 47 tiny bastards inside), 4 shamans was very agressive – they moved forward to constanty cast on my guys even chase them to have them in spell range.