A backup you have not restored is a hope, not a backup. We learned this the hard way on a 2018 ransomware case where our nightly snapshot turned out to be missing two of the three database volumes.
Since then every managed client of ours gets quarterly restore drills. Pick a random night last quarter, restore the whole stack into an isolated VLAN, and try to log in as a normal user.
The first drill is always the worst. Things you forgot to back up. Things you backed up but cannot restore without a license you only have in production. Encryption keys.
By the fourth drill the team has built up muscle memory and the gaps are small. By the eighth, restoring a single mailbox is a five-minute job, not a Tuesday afternoon project.
The dollar cost of running drills is modest. The cost of finding out your backup is broken on the day you need it is not.
If you take one thing away: schedule the next drill before you finish reading this. The next one. Then the next. — Chad