When backing up my catalog I get "Catalog size is more than than 4GB" (I thought this was fixed in LRc 2018). I click click continue, and after a fair bit of time it fails. When I watch the backup folder, I see it create the Catalog and data file, it then creates a temporary zip file which changes size. Finally after a while it fails and deletes the backup folder.
(As a work around, I duplicate the backup folder before it fails, and zip it up myself which works).
OSx 15.2, LRc is on a local drive mirrored to a Synolog Drive, Image files are on a Synology NAS, Catalog contains about 300,000 images.
Good point, the LrC catalog doen't contain images, only data about your images (and if you make the mistake of not using sidecar files, your edits), therefore, backing up the catalog is not a backup of your images.
Backup your images as well as your catalog (raid is not backup solution), you need your files in three locations one on a different site.
FWIW, the entire LR backup is kind of silly. You are only backing up the catalog (and zipping it), and not the images, presets and other important data. You'd be much safer turning this all off and using an actual backup utility to back up all your data, or data you can never afford to lose. Just the catalog isn't really enough.
Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"
I find LR Catalog backup useful as I have had Catalog corruption mutiple times (like you say, Adobe is not used to someone using a catalog with this many files). I just unzip the most recient catalog backup and drop it into my catalog folder and I am back up and running. I can't do that with an actual backup of an open catalog. BTW, I only backup my zipped backup catalog as a "safer" "actual" backup is often in an usuable state with lots of temporary files.
If you don't use (and backup) Adobe's backup regularly, you risk loosing your catalog which could include your edits (I recomend using XMP sidecar files to avoid this). Additionally, the act of backing up your catalog, optimizes and tests the integrity of your catalog. I currently run catalog backup everytime I quit.