i find enemies affected by the gas to be very distinct: no other effect gives them the smoky green aura. at to your point about it blocking targetting: this is a design choice that’s similar to globadier gas / sienna’s fire / IB flamethrower stuff. it’s realistic in that it’s gas and obscures view. imo not anything game breaking.
as to your question about it underperforming… take a look at the comments in this thread.
the hagbane is a crazy boss wrecker. in fact i would go so far as to say it’s the best boss killing weapon with a str pot and duration trait.