First of all, I want to say thank you to the Nuki team, that they answer and explain how they work/why they don´t answer usually to feature requests. That is really customer focused and sadly not standard today.
Yes, you are right, there are some possibilities that could end in an unwanted opening. However, if you calibrate the RSSI threshhold, then it should work very safely in many homes, where the door is not directly next to the living room. Furthermore, as I mentioned above, the current auto-unlock brought me into unwanted openings (and twice the door was open for a whole day because of that) many times. So to be honest, I would not call it very safe. But for me, that is not a huge problem, I mean if someone wants to enter the house, then he can do no matter what I try to prevent him from. And Tesla cars work exactly that way, so I think that this would be justifiable without further security checks other than just RSSI-range-check. I mean maybe there are homes and conditions that this feature would not work properly, however there are also conditions where the classic auto-unlock does not work, too. So if I could choose as a costumer which way I want auto-unlock to work, that would be great! 
And there are currently so many problems with iOS 15 regarding the geofence, that this feature would be great to eliminate the need of geofencing at all.
First of all it takes much longer to push 6 buttons instead of one, furthermore there is quite some delay after the code is fully entered and lastly if I have my hands full, it´s much more easy to press one “big” button instead of 6 small ones.