I needed a cheap backup server, and since I had a raspberry 3B in one of my drawers, I chose to use it instead of buying a backup solution.
The main advantage of using a Raspberry is that it consumes little energy and is fully configurable. The solution therefore remains very flexible and scalable.
The main idea is to use NextCloud to duplicate my work data, which is cheaper than using a NAS with a RAID 5 or raid 1. Here a small 2.5 HDD do the job.
In addition, Nextcloud has interesting features in the context of access for customers, for example. You just have to declare the server in the DMZ of your router to provide access from the internet with a dyndns, for example.
Data takes a long time to copy. It took me several days to synchronize my TB of data.
I hope you find this useful, and I wish you good make =)