I'd just say Robo's "mouth" plates look a little organic, as if they're sagging a little. Not sure if it's possible to make them more angular, or if that'd even be correct, since yeah, that's pretty much how Robo looks.
I'm absolutely thrilled to see these, though. You're wonderful at taking the Chrono Trigger portrait style and redoing it. These feel right at home in the CT ROM.