I've had some pretty poor experiences with offline maps on Google Maps. There have been a few times where I've made a point to download the maps for an area a couple of days ahead of time because I knew service would be spotty. I would then drive out to the areas with spotty connectivity, and more often than not Google Maps would refuse to generate a route for a route definitely within the confines of the selected offline map. I'd have to drive back a bit by memory to find reception and then try to generate the route again. This would even happen if I was on a route, cancelled it for some reason, and then attempted to restart the exact same route while along the route it was just on.

Google Maps offline has been extremely unreliable for me and mostly unreliable when I had the greatest need for it. This was true years ago and this was true less than a month ago.

If I'm going to someplace with spotty connectivity, I make sure to put in all the main addresses and locations on my car's navigation system ahead of time. The maps might be dated but at least it'll generate a route somewhat towards the destination, even if I might need to take a detour along the way.