They’re analogous to the far right is the main thing. Anarchism/communism/etc. is the gateway to such views. Most lefists don’t go that far (good) but some do. Same thing with the far right, they start off as libertarian, ancap, or run of the mill conservatives etc. and end up going into cuckoo land after they watch too much cable news and facebook conspiracies.
i don’t think there is a reputable source to substantiate this.