I used to mount my iPhone on a Delta mount, which is very secure and sturdy but not as elegant as the Quad Lock.
But since getting a larger Android phone last December it's always on me, usually in a pocket -- jersey, jacket or pants. I hit by a car while riding last spring and I was glad to have the phone on me in case I was separated from the bike. That can happen very quickly when witnesses or rescue personnel show up. You may not see your phone or bike again for awhile -- although in my case they put my entire bike in the ambulance with me!
And no problem with the phone slipping out of my pocket or being damaged when I hit the pavement. My helmet was dinged and shoulder was broken but the rest of me was okay, including the phone. As soon as I was able to stand I used it to snap photos of the surroundings, car and driver who struck me, etc.
I used to carry a flip phone in my pocket and never had a problem in the three times I crashed unassisted by outside influences.
If I carried a spare phone it would go in my seat bag. I no longer mount a phone on the handlebar. I got a good cheap bike computer for that -- much smaller and lighter, and does what I wanted. If I need to consult the phone for orientation I need to pull over anyway. And I can still hear the phone prompts for Strava segments and voice feedback from my cycling apps to tell me average speed, etc. So I don't really need it mounted on the bike anymore.