Beyond the colour of the helmet, there’s not much to differentiate the figures, although the Kader MIHK figure can feature a more menacing concave shaped visor.
The belt/torso section in particular deserves attention as mentioned previously. The rope belt is a completely different sculpt for each factory.
Unitoy ERG has some sculpting variations noted in the graphic below. There’s been two minor cavity variations noted in the arms, legs and and torso. These parts are mixed across the ERGs produced at this factory from HK through to the NCOO figure.