SIGMA Duo Magnetless Speed Sensor

I bought this speed sensor (one for MTB and one for road bike) to improve the accuracy of the indication given by the ROX11.1 GPS only. They don’t work well at all. The speed jumps continuously even +10 +15km/h, especially uphill and at low speed, which is quite impossible in the real world ; because of these continuous jumps also the indication of the slope is unreliable (always lower than real). The total distance is not accurate. Note that the sensors have been updated to the latest firmwre version available. In the last MTB ride I was riding at a constant speed of about 6-7km/h with a slope of 12-13% and the display of the Rox 11 continued to show a speed ranging from 7 to 25km/h...simply impossible, I understand that there may be tolerances but this continuos jumps are not acceptable. I did the same climb with GPS only and the speed indication became much more stable. I redone the same ascent with an old VDO M6 and magnetic speed sensor shows a constant speed and a costant slope as is the road actually. For me, these magnetless sensors are useless in the real world, maybe because they are not immune to the vibration of the bike. Even if inaccurate it is better to use GPS only speed indication. I do not know if it is a specific problem of this manufacturer or if it is a general problem of the magnetless speed technology. Manufacturers should inform customers if there are any restrictions on the use of their products. For me it was money wasted.