1) Airplanes are not exactly guns. One bandit comes along, wielding some kind of dual-blade, and kicks Kai's ass. No advanced technology for that feat. He wasn't exactly dogfighting Kai in that airplane.
2) As I mentioned before, fighting armies is not comparable to peace-keeping. One non-bender mook with a blade is not comparable to advanced killing droids. Qui-Gon and Obi-Wan didn't just flee because there were droidekas, they fled because back-up was on its way, and even they can't fight a goddamn army on their own. One mecha-tank is comparable to a droideka, if we're considering the powers of benders vs. the powers of Jedi.
3) I really recommend that you watch the clip above your comment. Sure looks an awful lot like Republic City to me.
Earth Kingdom? "That was nothing more than your assumption".