I have not had any issues with Kopia so far, but I have also only used it for maybe a year? My main reason for trying it was that I wanted to be able to give something to family members to use as a backup client with a reasonable ui. I can also control the default exclude list and default policies for compression/etc pretty easily.
I don’t know how many years of restic backups I have, but I still rely on it for my most important data. Anything really important on my desktop/laptop gets backed up via kopia, but also gets copied (usually via nextcloud) to a server that has hourly zfs snapshots and daily restic snapshots. Both the restic and kopia snapshots get stored on a local nas and then synced to rsync.net.