Very different products and can't compare them. Veeam is enterprise-grade and used for a larger variety of mission-critical workloads. Kopia is meant for end-user backups (though folks use it for a bunch of other things too).
Unfortunately, this is not true. You need to grab all the DB files (WAL, etc.) in a consistent manner. You can't grab them while writes are in progress. There are ways though. Look at what Kanister does with its recipes to get consistent DB snapshots to get a sense of the complexities need to do it "right."
I haven't looked at duplicati in a while and, it has evolved. While Duplicati's feature set looks similar now, I would need to benchmark it both for efficiency and final backup sizes.
And, while not directly, I know a number of companies, including mine, do test restores all the time.
I've been using Kopia for my personal use and for products I have helped build at a couple of enterprise backup companies! It's also used by other open-source backup projects that focus on specific ecosystems (Velero and Kanister for Kubernetes, Corso for Microsoft 365 backup).
I am obviously biased but it's pretty amazing. AMA.
Note that Veeam contributes to Kopia - https://www.veeam.com/sys451