Years ago, I got a backup phone when I spooked by the negative reports of the OnePlus One. All these turned out to be true and I experienced them (except for the Tech support). Luckily, there seldom if ever I need to to use the backup.
The phone that I got was the original XiaoMi Redmi. Those were the days of flash buys. Hitting F5 to pull the trigger. For the uninitiated, the Redmi is an entry level smartphone, ie lightweight specs. It can do the basic functions well. Remember this was before the popularity of WhatsApp.
Later when the IT Security started putting the screws on the users, I somehow managed to put Mobile Iron and Email+ on it. More luck than skill I think.
Recently after 3 years and some months of hard use, the OnePlus One is very slowly dying. One Sunday, the touchscreen digitizer gave up. I thought maybe the backup phone can save the day. I had to wait for the next cycle of backup and restore before starting.
The backup file was 627 MB.
The phone free space was 62MB. Tadaaaaa!
The lesson here, make sure the specs of the backup phone is close to the main phone.