1) The web app GUI is bad - can't reorder saved options/backups/etc, each of these options use up too much screen space and there's no compact mode or minimising the options, it could be more user-friendly especially with being able to add arguments with the GUI rather than text.
2) It's proprietary with source-available but not open-source. This makes me hesitate to rely on it in the long-term as I don't know if it'll remain supported, especially considering that:
3) Development speed was very slow when I dropped Duplicacy about 6 months ago, and seeing Borg have many contributors makes me think it's more likely to stick around in the long-term.
4) GUI doesn't give much insight into the status of the backup other than progress %.
5) Restore operations are easier and quicker with Borg/Vorta because you can mount the backup via FUSE.