Hi Dimitris. I’m not sure exactly how to answer your question, but to my mind the very essence or definition of “functional” training would tell me that this is an appropriate form of exercise for EVERY client. The real issue comes down to how much, or to what degree we use that method of training, and the answer to that question is very ‘client-specific’ (depends on the client’s needs).
I hope that this helps.
Any training the elicits neuromuscular adaptations is going to be functional. Whether your improving motor patterns, or increasing lean muscle tissue each is going to be functional to the human body. People misinterpret functional training. The movement of the human body is functional and exercise training to help contribute to more efficient movement will result in a functional outcome.