I think that this is very much about context. I have a lot of CCTV covering my property but except for one camera its recording is limited to the boundaries of my property. The one camera that does have a portion of my neighbours driveway, my neighbour is aware of and he knows what is being recorded, which he was grateful for when his vans got broken into. I also install quite a lot of CCTV and have refused jobs on the grounds that the install would be compromised as it recorded too much street-scene or other people's property. I have also installed covert CCTV for prosecution purposes - but this was heavily documented and regulated. I have never known the Police to become involved in any CCTV disputes as that is normally the domain of the ICO or Civil courts.
Smart doorbells are just a way of life and given that many Police forces actually had / have a program of giving them away in high crime areas, I highly doubt that the legality of their use, in a normal installation scenario, is going to change.