What do you mean by "secure"? Like "won't get data loss or corruption", or "secure from unauthorized access"?
Regarding the former:
You should make offline (i.e. not connected to any systems except when being backed up) backups... Google is very good at not losing data, so there's probably a far greater risk that your account gets compromised on your end, and your data wiped or made inaccessible because of that.
Other than guarding against that, I dunno that I'd bother with further backups - for secure data, you need to have offsite copies (in case of catastrophic destruction of location where data is kept) and some kind of system to maintain file integrity (e.g. ZFS or SnapRAID). That's fairly ponderous for a home user to setup, but Google already does all of that.
Regarding the latter:
Google's pretty high-security with their data centres and fairly resistant to social engineering attacks, but the data isn't encrypted on their servers. Regular Joe has nothing to worry about other than losing account access due to their own poor security practices. (Re-using passwords with other accounts, not using 2-factor authentication, installing keyloggers, etc.) If you're doing anything that could run you afoul of the government of the country you reside in, or of any other highly-resourceful nationstate or organization, you should be using open source tools encrypted cloud platforms. (I like SpiderOak)