With the hollow back I mean there's some clearance behind the Other Half and the main phone, so the whole device isn't as sturdy as it could be. Also tapping the other half with your fingers makes kind of hollow sound. It makes the phone feel little bit plasticky and cheap. But like I said, it could be fixed by stuffing the space between main phone and the other half with some kind of filling, for example Blu-Tack. Same fix could be done by squeezing tiny bit of it on both sides of the volume button to prevent it from moving around sideways.

I hope people at Jolla consider these issues when planning the next HW version. Or they could ship the phone with a extra piece of Blu-Tack for modding purposes