This is not particularly risky. Just make sure that you don't keep your screen ON for long times.
Most speedometer apps will have this issue. While using speedometer apps, other than the number (speed) being displayed, the rest of the screen does not change. If the screen is left ON for a long time, this can cause issues. If your phone has an LCD screen, then this is not an issue at all. If it has an AMOLED display (like all Samsung phones do), then you 'may' have to be a bit careful. But if you are not keeping your phone screen ON continuously, then it is not an issue at all. Also, the app can run...
more... in the background, and show the speed in the notification bar. So, it's perfectly safe to use.