I have recently bought a My Cloud Home Duo after having received confirmation from WD that it supported macOS High Sierra.
To my surprise, I found that Time Machine (abridged TM afterwards) backups are still mounted through Apple Filing Protocol, which has been superseded by SMB2 as the default protocol for sharing files starting with OS X Mavericks (10.9). WD must be using some sort of hacking to use AFP for Time Machine because Apple said that, when using APFS (which is mandatory with SSD under High Sierra), TM is only supported over SMB Shares (See: https://support.apple.com/en-us/HT208018.
APFS and Time Machine
You don’t need to change any Time Machine settings to back up APFS-formatted disks.
Any Time Machine share points must be shared over SMB instead of AFP.
Actually, TM over SMB is supported since Sierra (10.12) and Apple published a specification on 13 September 2016 (see: https://developer.apple.com/library/content/releasenotes/NetworkingInternetWeb/Time_Machine_SMB_Spec/index.html). To my understanding, it specifies extensions to the SMB protocol to ensure reliable backups.
WD introduced TM over SMB on other products, see for example My Cloud OS since last September, so it seems that WS masters the implementation.
Finally, Samba 4.8 released on 13 March 2018 is said to support TM (over SMB). Samba is an open sourced implementation of SMB that it used by WD and others.
Looking into the GPL softwares used by WS, My Cloud Home seems to keep relying on Netatalk to run AFP file sharing and which to my knowledge is only faking the support of the required features to ensure reliable Time Machine backups.
Apple says APFS hard drive requires SMB to run Time Machine today, so it will be in the future. I would like WD to confirm that it intends to support TM over SMB soon for My Cloud Home (Duo) (and that in the mean time, Time Machine Wi-Fi backups are reliable with My Cloud Home).