For Epic Skeleton mode rigs, some twist bone controls might not work or work unexpectedly. This is mostly limited to location and scaling. Rotations work the same as rigify except for the first twist bone which is a special case.
For Free Form mode, rigs match the original rigify rig almost entirely. But there may still be minor differences.
IK Stretch might cause issues with child bone scaling in all 3 directions instead of just 2.