Most of the folks who open feed are probably feeding an outyard with several hives. A small percentage will be taken by ferals. To me, no worries. I dont mind helping them along a bit. Spreading disease? It is probably going to happen whether you open feed or not. Drifting, robbing, drones switching hives... Everything we do, including putting the bees in boxes, brings an issue that the bees have to deal with, different from what it would be in the wild. I open-fed last year with 5 hives. During warmer winter weather, i fed 3 parts sugar to one part water, in the bucket. All survived, dispite my ineptness.