They are all we hear about during the floods, as though they are some kind of magic bullet to prevent flooding - they are definitely not.
Filled and placed properly they can reduce the flow of water, but they will not prevent seepage. Once in place they have to be monitored continually with constant mopping, pumping or bailing and that includes on river banks, roadways and homes. They cannot be left in a doorway in the hope of stopping water getting in. Unfortunately that falsehood is what has been sold to those demanding them. They are not dams, they are, at best, temporary restrictions. They also will do absolutely nothing to stop rising groundwater, which is one of the biggest problems people are suffering.
Another thing that I have noticed about those being used is that they are overfilled and not tied in properly. Two-thirds filled and loosely tied are best, and then overlaid in the way brickwork is. Some of the plastic bag monsters I have seen, built in columns, are worse than useless. They are not much more than the visible "help" people have been led to believe they need.