The exact words confuse me a bit. Downloading is easy. Even Google Drive’s web UI can get raw files. Assuming this is a broad question of “does anything ever go wrong?”, unfortunately it sometimes does. Duplicati (or the network, or the destination, or something) will create some problem with restores, and
Duplicati.CommandLine.RecoveryTool.exe
This tool can be used in very specific situations, where you have to restore data from a corrupted backup. The procedure for recovering from this scenario is covered in Disaster Recovery.
Independent restore program and other tools can sometimes help, but it’s difficult when things go wrong.
This is one of the tradeoffs of (mostly) the block-based deduplication which saves storage space by not having direct copies of source files, but treating them as blocks which are deduplicated and then stored.
Block-based storage engine (simplified explanation)
How the backup process works
How the restore process works
I’d think if you downloaded all files from Google Drive somehow, one of the recovery tools could probably find some files to restore despite damage, but there’s not a one-to-one map from remote files to source.
Best practice is to not rely completely on a single backup program, if one wants least chance of file loss. Duplicati is still in beta testing stage partly because people occasionally find issues, and they need fixes.
Especially worrying to me is the people who backup, delete the original, and rely completely on backups. Try to keep yourself out of a position where you would be in a really bad spot if some backup had issues.