Systems theory, like catastrophe theory before it, is a descriptive theory not a predictive theory. Which, means that it’s harder to say if it’s any use (and, indeed, you can always re-phrase any discoveries within that framework using the language of the old framework, once you have made them).
Given this, we’d expect the most utility of systems theory to be in fields which are suffering most from a lack of adequate epistemological tools. Which is why, I guess, I’m convinced of the necessity of some kind of systems thinking in cognitive neuroscience and for social psychology.
And why, maybe, to date the best systems theory work in psychology has been in developmental psychology