They are considerably more complicated to build and their main benefit is efficiency in turbulent flows, however it turns out that also turbulent flows tend to be slower in practice so there's less energy to get out of them anyway. This reduces your margins to zero if not negative unless you scale them up to sizes that are not practical for places that actually do have turbulent wind conditions. (e.g. cities.)

Source: I worked for a micro generation helical VAWT startup for 5 years.