@skipcave: If the power outage etc. leads to a failure where Insync database gets corrupted, then, Insync cannot start from where it left off. Database corruption happens in rare cases though.
Yes, using Insync, you may selectively unsync once the files have been uploaded, this way files will get deleted from the local folder but continue to remain on Google Drive.
https://forums.insynchq.com/t/how-to-selectively-sync-google-drive-folders-and-files/31