I have Office365 Home edition (~US$90/yr), which allows up to 5 family members use the apps, which works for me. OneDrive has 1 TB of storage. There is a free Office edition for students, so buying a license may not make sense.
As someone stated earlier, I do not use flash drives for storage as those can be lost fairly easily, so these are only for transfering files when needed.
Another concern would be ransomware, which sadly is more rampant now. Having your files corrupted on your computer hard drive would mean your backup would also be corrupted if it syncs automatically (it should).
Windows 10 has ransomware-protection features which prevent the OneDrive folder from being accessed by other apps.
In the past I backed up on an external hard drive, but those can die too. OneDrive is easier for me since I don't have to worry about synching files. I can always copy files to a hard drive ever once in a while just for peace of mind if needed.
There is also the issue of encryption. Windows 10 Pro includes BitLocker which is a nice feature since the hard drive is not readeable from another computer(you want to backup on a cloud service for this). I don't know if Google Drive has any encryption features as I have not used it in a while, but it's a feature that I believe is coming later this year for OneDrive.
To recap, with cloud storage, I really don't need an external hard drive/flash drive.
Just my 2c,