I've built kits, and I've torn down and rebuilt RTR set-ups. Best Kit I ever built was the Mugen MBX5R, Best RTR I tore down was the Revo Plat. But of all the RTR I have torn down and been disappointed with was the 4-tec. Nothing like finding over 10 stripped screw holes on a RTR that's supposed to be able to do 70+ MPH. With a kit most times you get upgraded parts, metal or carbon fiber vs plastic. And the carnal knowledge of the inner workings of your machine.
I wish EVERY RTR came with a KIT Option. But it doesn't work that way.
Just my 2c
Mike