Those criticizing their use argue that they don't accurately represent the stream bottom and are biased toward or against certain types of critters. Those favoring their use argue that they rigorously standardize the habitat, are more quantitative, allow sampling of deep rivers, and, for certain applications, enable the investigator to sort out habitat effects from water quality effects. I tend to avoid artificial substrates unless they are the only practical sampling method or are being used specifically to assess the impact of habitat degradation. Most sampling devices are geared to rocky bottoms. The operator reaches down and disturbs the streambed within the round open bottom of the sampler.
Water flowing through the front of the sampler carries the critters into the net.
Sampling non-wadeable streams remains a challenge as all the above devices are designed mostly for use in wadeable streams. In either case, the samplers are used from a boat and lowered to their location in the water column or placed on the bottom. Frequently a stream reach contains multiple habitat types. Not surprisingly, biologists argue about this too. Others believe that sampling all habitats better reflects the true condition of the biological community.