They do work on small underpowered boats and motors. As a machinist I have made a few over the years for small out boards that actually worked quite well. Today, many folks have a poorly balanced boat and motor combo, improperly setup and expect an add-on fin to cure there problems. Many folks see an improvement, even though they could have made a bigger improvement by just creating a better setup ! Often these are installed on high powered motors only to have the increased water pressure cause actual damage to the motor itself. O me most gizmos on the market, there is a time and a place for them, but mostly an excuse for an improper setup.
