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.”
Strength stabilization can be better achieved with unstable surfaces.
However, as Natalie mentioned, pure strength should be done on stable surfaces to provide more safety.
As an example, people bench pressing should ALWAYS keep their feet on the ground. Last thing you want is to drop too much weight on your chest.