Cubot have released a video, where they are trying to compare the accuracy of heartrate monitoring of their Cubot V2 smartband against the direct competition product with Xiaomi MiBand 2. Both smartbands are pretty similar, but the difference is in the method of the heartrate monitoring itself.
While the MiBand 2 can only do the single instant heartrate test, Cubot V2 supports dynamic real-time heartrate monitoring. The video comparison itself looks fairly objective with several test subjects of different body build and the control measurements are done with a professional heartrate tester.
Of course compared with the professional one there will be some deviations in the results taken by the smartbands, but seems like the Cubot V2 is the one with more stable and accurate monitoring. After all check out the video and judge for yourselves.