Hi Massimo, I have had my T-REX 3 since last October. It is true that earlier Amazfit models had inaccurate heart rate monitoring.
This is my third Amazfit smartwatch and it is now finally very precise. I have checked it against a chest-band while training on my Tunturi ergometer: there is no need to wear the chest-band, the watch is very accurate.
I may add that the GPS is starting up super fast compared to earlier Amazfit models and that the distance-measuring precision is high.
Finally, the battery life is unreal. You can travel overseas for two weeks and do some walking and running and then returning with a 30% charge!
I can highly recommend with conviction.