Generally speaking CVT transmissions are reliable, although we would suggest looking at the 'What to watch for' section of our reviews for any models you are considering where current owners submit issues they have experienced themselves.

As with any kind of modern automatic transmission, it is designed to respond to prevailing road conditions and many will detect that you are going downhill and automatically engage a lower ratio to provide engine braking. Also many offer a 'Low' or 'L' position so you can manually engage a lower ratio. When ascending the gearbox will automatically shift down to ensure sufficient progress.