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.”
The SAID (specific adaptation to imposed demand) still applies, and if my goal were to make somebody as strong as possible in any given exercise, I would not opt for an unstable surface. if I were sure that the person already had very strong core muscles.
However, in the process of getting him/her there, I like unstable surfaces and usually see improvement in peripheral strength just by strengthening the core. Personally, I like to include unstable apparatus for it but Pilates and other modalities are just as valid.