I do not know if I can post it here but what troubles me on Rigify is shoulder problem. The twist is not compensated there by default(is it possible to tune this anyway on some part of the rig???) so the shoulder bone(namely: DEF-upper_arm.L) is fully rotated along its Y axis and the deformation is not acceptable there. The volume must be kept with influencing the X and Z rotation BUT the Y rotation must stay on zero or slightly bigger value(the custom attribute to magnify this would be good aswell) to fluently smooth twist rotation between shoulder and elbow joints along the humerus.
Thanks.