1. The magnet does not hold the blade firmly enough and there is some wobble allowing some small variation in the grinding angle. It is better with minmal to no pressure.
2. The discs are not durable and they collect sharpening debris which needs to be cleaned. After only a few uses I can see there is less material being removed and the sharpening speed is slower than when new.
It does make knives sharp but it takes more time, care and maintenance than you would think.