There are a number of caveats though.
Systematic reviews are secondary research ('research on research'). The object of scrutiny is not the incidence of schizophrenia per se, but the literature on this topic. So while this type of research cannot be used to 'prove' a hypothesis, the compiled data can encourage the generation of new hypotheses that can then be tested prospectively, with new data.
National or health district catchment areas often provided convenient boundaries regarding access to services and minimal out-of-area leakage, but they may not be optimal for detecting variations of the disorder within or between various populations.
Most of the studies provide rates that are the 'treated incidence' of schizophrenia (ie, the count of individuals who are identified by services and treated); this probably underestimates the true (underlying) incidence of schizophrenia.