- The greatest advantage I see with FK is ease in achieving overlapping action, e.g. between upper arm and forearm.
- Natural Arcs while interpolating between keys.
- When elbow is planted or when we just need to move forearm then FK is easier.
- The best thing about IK is the simplicity it offers in creating a pose. For example, positioning a hand with IK is very intuitive and predictable.
(With FK, I believe the problem is that you have to think in terms of two rotation transforms to make hand reach a goal position. Also, Euler rotation introduces a certain level of difficulty if axes are not in proper alignment, i.e. two axes are closer to each other.)
- Animators have to deal less with Euler rotations, hence less Gimbal lock problems.
I understand that many times FK or IK is a personal preference. So a particular behavior of FK/IK may be difficult for one person, but easier for another. But my goal here is to list mainly logical advantages of each system.