About open feeding being an invitation to the neighborhood bees, who may come
complete with all manner of problems.
 
Let us not forget that one form of open feeding is a nectar flow.  I very
much doubt that a field of bloom will be monopolized by only one apiary if
there are others within flight distance.
 
Personally, I do not do open feeding because of the robbing potential and the
difficulty of controlling the feed input to individual colonies.   But I
wouldn't consider open feeding to be any more of a disease/mite spreading
risk than a blooming locust tree.
 
W. G. Miller
Gaithersburg, MD