How often do we catch a fish, early on, probably first cast, and then struggle. To me it's poor water craft, and I have not just taken enough time in settling the feeding fish before introducing a hook bait. I might get away with than on a bigger River like the Severn but not on say the Warks Avon. Of course regular feeding will concentrate the Barbel's minds, and will be less likely to spook. However on pressured waters the very act of 'feeding' can ring alarm bells, and for me this is where the single bait and wait approach can score.
I have seen Barbel scatter for their lives on the Teme, when loose feeding corn, and the visible corn will remain untouched, but will be mopped up in the night. Hemp is less likely to spook, as I suppose is small amounts of small pellet
Of course all the above can be nonsense, and probably is, because there are always times when the rule book is torn up and for no apparent reason you can't stop catching, despite making no real efforts as far as 'water craft' is concerned. One particular session I recall, (they are indeed rare) I was fishing the Warks Avon, doing nothing but loose feeding 14mm halli pellet and using the same on the hook, with a lead, and in a couple of hours I had six good Barbel and six good Chub, in the afternoon in bright summer sun. The fact that it was the days when there were a good head of Barbel in the river must have made them very competitive.