About three months I returned from a mission, being district leader for about seven months with sisters in my district. I would have to agree that they are often polar missionaries; they are either the best or the worst.
However, in my case they were impossible as a leader to counsel or correct. Sisters can basically do anything they want and no one says anything because the mission president has to protect them. With an Elder the mission leaders can have a talk and sort things out, but with Sisters that can't happen because of provisions in the Missionary Handbook.
Except for one case all the problems in my district were from sister missionaries; they were either lazy, murmuring, or they expected handouts from everyone. When an investigator had a doubt or concern, Sisters generally are not as agressive at solving the problem; they give up too easily.
The best thing that the Church ever did with Sister missionaries is instal them in places like Temple Square or Nauvoo, where they don't have to interact with Elders.
There have to be good Sister missionaries out there, but I have yet to meet one.
