Here is the good news: it is indeed possible to achieve good accuracy in HRV measurements using this technique, but the methods needed are slightly more complicated than acquiring a video and computing peak detection on the PPG signal (which is sufficient for heart rate measurement). This post covers the steps involved in the implementation of Camera HRV, the iPhone app I developed to measure HRV using the phone's camera. The algorithms are part of HRV4Training since version 3.2.
Later in 2017, we have also validated these methods and published the results with respect to ECG and Polar straps, showing equivalency. You can find the full paper here.
1 - Data acquisition from the phone's camera
2 - Filtering & smoothing
3 - Resampling with cubic spline interpolation
4 - Peak detection
5 - Artifact removal and features extraction
6 - Comparison with heart rate monitors (Polar H7)
7 - Tips