Vis
You can get the situation where the visibilty is poor and yet there is no cloud, industrial muck. In Germany many a day was 1km vis and no cloud. Do you include cloud cover with the limitation.
The limit is not the only problem as they may set off in good conditions which then rapidly get worse. It is knowing when to say enough is enough! Either turn around and go home to my loving family or spend the night in a hotel close by.
Major problem will be educating ppl pilots that they are in a helicopter and if the weather deteriorates below what they consider comfortable then land. Thats what a helicopter is good at, does'nt need an airfield.
The old sayings "Better to be down here wishing you were up there than up there wishing you were down here" and
"If there is any doubt there is no doubt". Land the bloody thing it is a helicopter.