Time Machine and Google Wifi and WD My Cloud Home

WD MyCloud can work.

Instructions here.

https://support-en.wd.com/app/answers/detail/a_id/2374/kw/time%20machine

The reliability of Time Machine to MyCloud is somewhat uncertain. I would certainly consider alternatives like Synology NAS which have tested long term and work well. The issue is most testing is not done over long enough periods to actually get reliability stats. I have tested other brands .. not yet a MyCloud and found it works fine for a few weeks but after that time became corrupt and so the TM backup was continually failing and needing to be scrubbed and restarted.

Have not personally used either of the products that you ask about, but the Google WiFi should work with virtually any WiFi devices. My neighbor is using Google WiFi for his Macs, iPhones, iPads, printer, etc and reports no problems at all.

However, the My Cloud Home will not be compatible with Time Machine if you are planning to back up your Macs that way.......unless......the My Cloud hard drive can be reformatted in Mac OS Extended (Journaled).

You might want to check with the My Cloud support folks to see if it is possible to change the drive format, and if so.....how exactly to do this.

Any of the modern wifi systems should work fine.. security is not the issue. All the big names will keep releasing firmware updates that will fix any issues that arise.

The bigger issue is how well it handles Apple protocols. In theory this should not be a problem. Bonjour is LAN only and is essential for Apple's ability to handle network devices. Unfortunately the reality is not all brands work great with Bonjour and so you might find it has dropout issues. Finding the WD MyCloud across the network for example can be hit and miss.

See instructions for manually configuring the MyCloud.

https://support-en.wd.com/app/answers/detail/a_id/4785

The big advantage with Synology is they are simply a lot more compatible and have done a lot more work on fixing SMB so latest MacOS can do TM over SMB (now its native protocol) instead of needing to force the connection back to AFP.

Speed over mesh system can also be disappointing. Remember rules about double hop wireless causing large speed drops will apply to any mesh system without proper backhaul like ethernet or dedicated wireless channels like the Netgear Orbi or Linksys Velop higher end systems use. For most people internet speed will be fine but once you start trying to backup over a network you will find speeds are glacial.

Apple added TM backup over network between Macs at High Sierra.

https://www.howtogeek.com/330288/how-to-set-up-your-mac-to-act-as-a-networked-time-machine-drive/

That is useful if you have a desktop Mac .. otherwise Apple do not have a solution for you and I recommend the Synology over other NAS.