Nearly all of the research in this search says there is either no statistical difference in effectiveness or “The diminished force output suggests that the overload stresses required for strength training necessitate the inclusion of resistance training on stable surfaces.”
Good insights. I think that, in general, especially for the beginner, untrained, or someone who is simply interested in achieving and maintaining balanced major muscle group strength, training on a stable surface is the best option.
For top level competitive athletes, I believe that the research suggests that training on a sport-specific unstable surface offers the advantage of enhanced balance, not strength.
Take care, Daniel