If you disable Remote Access, from the My Cloud Dashboard and disable any port forwarding and UPnP to the My Cloud in the router then your broadband firewall generally should prevent any intrusion to the My Cloud from the internet side of your broadband connection. Then you pretty much just have to worry about someone either hacking your WiFi or your broadband firewall or a wired Ethernet wire to gain access to your entire local network.
There has been some discussion on some potential security issues with the My Cloud in the following threads.
https://community.wd.com/t/latest-firmware-still-vulnerable/96743
https://community.wd.com/t/reporting-security-vulnerabilities/96387
https://community.wd.com/t/how-secure-is-wd-my-cloud/95914
https://community.wd.com/t/where-to-report-vulnerabilities/95898
