I am running backups of our VMs (about 7 to 8TB) on two Repo's. One is a local StorageSpaces "RAID" with a ReFS/NTFS partition and the other one is an iSCSI target on a Synology NAS.
Both Repo's are about the same size, round about 15TB each.
I had all the luck with the ReFS Repo and it failed a couple times, I have lost 2 to 4 weeks of backups.
Luckily I only used half of the partition and I was able to create a local partition with NTFS and it is working like a charm.
The Repo at the NAS is ReFS as well and it is working great to be honest, did not have one problem since I created it.
I still want to change the setting of the local RAID and replace Storage Spaces with a RAID created with the HP controller and a certain STRIP size that was recommended in the thread "REFS 4k horror story".
At the moment I would not recommend using ReFS, if you run out of luck you loose your backup data and that could cause some issues!
Maybe in the future I/we can migrate to a stable ReFS Repo and use it's benefits, like fast clone (which is really fast).