Xiaomi always scores between the worst heart rate monitors on the market, I don't believe they achieved anything they are claiming.
ECG will be horribly inaccurate, as it is terrible on Apple Watch, and blood pressure simply won't work for any reasonable accuracy that is not considered as a medical hazard.
Just wait for Robs review with measurement, he pretty much puts everything up to the real rest.
I have bought Xiaomi watch S1 and I think this is the worst electronic device that I had in my life, so I am not touching anything from Xiaomi anymore.