I get asked this alot too! (… and I like the word “challenging” rather than “difficult” myself)
Performing the plank on hands or forearms all depends on which area you are targeting and your physical capabilities. When the plank is performed on the hands the triceps and shoulder muscles are more activated while the body is positioned more vertically against gravity . So when a plank is performed on the hands, you’ll feel your shoulders and triceps more and your core less.
If your goal is to develop shoulder strength and stabilization then the elevated plank on hands is probably more appropriate. You might also find relief from wrist pain in doing the plank this way by holding in to hex dumbbells pointing toward the front edge which will allow the wrists to be in a more neutral position or use a wedge/pad to lift the heel of the hand enough to relieve pressure on the joint.
If the plank is done from the forearms the triceps work less and although this version open’s up the shoulder joint, the core is working much more due to the shift in center of gravity so if your goal is core strength then you should choose the lower forearm position