Google Maps estimates are wildly off when parts of the drive are along unpaved roads in Australia. It assumes a ludicrously low speed of 30-40 km/h for such roads, when most cars are able to go 80+ km/h, depending on the road. I've beaten Google arrival estimates by more than 2 hours on some drives.
When planning drives involving unpaved sections, we usually ask Google for the estimates for the paved sections (which are generally accurate), then estimate the unpaved sections based on the distance and a guess at a reasonable speed for the road.
I've also noticed that Google is much better at estimating speeds along unpaved roads that have mobile signal coverage. It seems like it uses user-generated driving data for them to some extent, but not at all for the ones without coverage. This leads me to think that they accept real-time user data, but will not "queue" the gathered data on the app for uploading later, when mobile signal is available.
This seems like a strange decision, given that unpaved roads and lack of mobile coverage correlate by nature. I suppose it's probably a security-minded decision, to prevent malicious agents from easily uploading bad data. Or maybe a quirk of the way Waze data plays into this.