Once an iOS build expires, you can't take a backup, so you can do that to get the phone working again, but you lose any un-backed up data on the device like photos and app data. In the days before iCloud it was a huge pain.
Which is exactly why they (still!) plaster it with warnings to that effect. If you want to avoid any headaches the advice has always been to use multiple devices and keep the beta off your day to day device.
Can't speak for the OP, but in my case, I had it happen twice between beta 1 and beta 3 when I tried to skip beta 2 because I didn't care that much. It was also an iPad that left upstairs and was too lazy to carry it downstairs to update. Not a primary device, so all I lost was some progress in a few games, but I can see how someone could have suffered the same fate with their main phone and have lost a lot more.
Which is exactly why they (still!) plaster it with warnings to that effect. If you want to avoid any headaches the advice has always been to use multiple devices and keep the beta off your day to day device.