Scott ended up winning Volusia by 10 points. Brown lost Clark by 2 points. This is why you don't poll single counties, ladies and gents.
I would point out that the poll conducted of bellweather counties in 2016 was very helpful in gauging the level of support Trump had in the rust belt that state level polls missed. It showed he had massive leads in Luzerne, PA and Sandusky Ohio and those proved to be accurate.
Regions would be better than single counties. Unless you're talking about some massive county like LA or Clark or something.