Data for apps which use Google Backup Service will also be restored, though only a small fraction of apps use this service.
I've read some say they've kept their data, but I've read others that say they got wiped.
That way you can wait or correct things (sync elsewhere, back up to paper, fix the corruption, etc…) before you upgrade.
Again, you could just upgrade and cross your fingers, but it's more about your data than the upgrade causing millions of people to lose data.
Normally, you can get your lost data back with the above three methods, and we have made a clear table to help you compare the methods, and you can choose the method you prefer.
Except losing data during i OS upgrade, there are still some though problems you may meet, like lack of free space to upgrade new i OS, get stuck during i OS download installation, stuck on Apple logo or other problems.
Next week Apple will launch the i Phone 7 with its new operating system.