The Xiaomi Mi6 features two rear cameras, on is a 27mm f/1.8 lens and the second is a telephoto 52mm (equivalent) f/2.6 lens. If you understand the technical terms, you probably notice a disadvantage here with the telephoto lens, it has a relatively slow f/2.6 aperture. This means that if you shoot with the telephoto lens, you'll be more restricted when shooting in low-light conditions. Furthermore, the telephoto zoom camera has a smaller sensor and smaller 1.0 μm (micron) pixel size, compared to the wide-angle camera that has a 1.25 μm pixel size. The reason I know that the telephoto sensor module is smaller is because both sensors have the same 12-megapixel image resolution, but according to the official specs, the wide-angle camera has larger pixels, so I conclude that the sensor is smaller. I assume that the reason for that is to keep the phone slim, as a brighter telephoto lens would have protruded and probably cost even more.
Just for comparison, the iPhone 7 Plus has a 28mm f/1.8 lens for the wide-angle camera and a 56mm f/2.8 lens for the telephoto camera, so the Xiaomi Mi 6 is actually slightly better in that regards, I don't know about the pixel size though.
If you want to shoot in low-light, it would better if you use the wide-angle lens for that, because its faster aperture would allow more light to pass through the lens and produce images with better exposure and will also not force the camera to use a higher ISO sensitivity due to lack of light gathering capability that this lens allows.
Of course it would be better if we could have the second telephoto lens with a f/1.8 aperture for better low-light performance. It's not a big deal when it come to Bokeh effect, because you the Xiaomi Mi 6 can beautifully simulate it by software. The only issue that can arise is when shooting in low-light situations with the telephoto lens.