We have two Razor Ground Force Drifter karts that my kids ride and I think they're great for what they are--cheap, simple, kid-sized karts that work pretty well. However, a weak point is the band brake.
I don't know what they felt like when they were new (bought on Craigslist, well-loved). I have yet to successfully adjust the mechanism to get a decent amount of stopping power. So my kids have gotten used to driving them without brakes for the most part.
I'd love to fit a disc brake but the wheels are so small, it's really hard to find a disc that's even smaller! So I can understand why they opted for the band brake but still, it doesn't seem to work well in my experience.
Random ideas:
Put a brake rotor on the motor
Add a second band brake
find a better band brake
make the existing band brake work better
Add a scrub brake--the rear "tires" are plastic on the Drift model
(googling I found this page http://www.diygokarts.com/kart-parts/go-kart-brake.html ) which included the clever idea of using the drive sprocket itself as a rotor--ooh, I'm going to look into this.
An exotic solution--change to a controller with regenerative braking. How challenging/expensive would that be?