It also doesn’t offer edge refinement controls. In part, that’s because the built-in FCP effect recommends a static background and a stationary camera. For example, in my tests Keyper generated a more complete mask.
Although Apple itself released a similar scene removal masking effect in a recent FCP update, there are differences in how each operates. Sheffield recently released Keyper, which is a plug-in that can automatically extract a matte for a person without using a green or blue screen.