I'm on chapter 9 right now, I think it's harder than the first one, but I've played the first several times so it's not that hard anymore.
It's kinda annoying that theres so many things attacking you at once. Like fighting 3 giant sword/missile launching flying mech things and having four rapid fire rocket launcher guys shooting at me from far away.
It also seems to make this one more difficult, they resorted to being cheap in more places. If anyone's fought the flaming armadillo boss thing, they probably feel the same way.