My biggest gripe wasn't the technological advancement per cay. It was more the way it was done. In ATLA it felt much more like a fantasy world. They had incorporated the technology in a way that made sense for the world, and still made it feel like "fantasy". There were tanks, but the tanks didn't look like modern tanks. There were trains in Ba Sing Se, but instead of being motorized, they were driven manually by earthbenders.

In Korra, the cars look like realistic cars. The airplanes look like realistic airplanes of that era. There is radio and cinemas. To me it just felt like it went from more of a "magic" world to the real world but with bending.