I guess that would depend upon where the weakest link in the individual’s kinetic chain. Whether one performs it on their forearms or with their arms extended doesn’t change the fact that it is a closed-chain exercise. The plank is a total body exercise. It is not just a shoulder girdle exercise and from reading the above comments it seems that much emphasis is being put on the shoulder girdle when so much more besides shoulder girdle strength and stability is being challenged in that movement. We’ve all seen stable shoulder girdle and sagging hips with this exercise.
From my perspective, when the core is stable and strong, and we all know the core represents all the muscles that attach to the axial skeleton, the appendicular skeleton can work more effectively. Hopefully, the musculature of the client’s appendicular skeleton is stable and strong so that it can handle the load that is being placed upon a longer lever (the extended arm) with shorter and smaller base of support (hand). Much more core strength, stability, balance and endurance would be required as the base of support has changed dramatically.