Genesis 8 model character is a good example for CG character, terrible for game characters, due to the fact that the arms and leg joints are chains, opposite to the Metahuman/Mannequin character, where the twist joints are attached to the main joint ( lowerarm twist joints are children of the lowerarm, upperarm twist joints are children on the upperarm, and so on ).
The only way I managed to get IK working with Genesis8 characters is by using virtual joints, so imagine doing this for every single character…and the twist joint setup is very annoying due to be a chain.
So long story short, Metahuman characters do have a twist joint setup, but as of now gimbal lock is still an issue ( I solved this via BP ), and the team is planning to upgrade the setup by avoiding gimbal lock with Control Rig.