I checked out Robo Army and Mutation Nation.
So for Robo Army it seems alright, but kinda stiff and not all that much better than Ninja Combat. I think I do like it more than KOTM2 though.
As for Mutation Nation, wow that really is a great beat em up. Feels great to play, decent enemy patterns and good pacing. Also, maybe this is just me, but it looks and sounds amazing. This game has a really excellent art direction and the animation work feels leagues above other Neo Geo games of its age.
I can see why yagamikun will never part with his Mutation Nation.