That is correct - no cell signal means no live updates. Typically, Apple and Google both seem to "stream" data to your phone for a period of time (i.e. if you have a route that takes you 4 hours away, it likely will have the route "pre-loaded" for a period of time; anecdotally I've seen this work on my iPhone for about 20-30 minutes after losing signal before I hit the edge of the map; I was back into a signal area not long after). So, it's not like a light switch where as soon as signal drops off you are suddenly without navigation as there seems to be some buffering in my experience, and thus the mapping software does seem to compensate for momentary drops in cell signal from time to time.
I think the cons of a built-in Nav system are far greater than the cons of relying on CarPlay/Android Auto. In my limited experience, built-in nav systems seem to be constantly out of date, and they all seem to be designed with minimal User Experience stuff in mind - for instance they often require very specific syntax (i.e. "1234 50th Ave" gets you to a totally different city than "1234 50th Ave E.", even if there is no 50th Ave W. in the city I'm in!). I also know of quite a few that would require you to put in the name of the city, the zip code, etc. before getting to the address; if I'm nipping across town to an unfamiliar store, why do I need to tell the machine that my intended destination is in the same city using 4-5 clunky menus? An aftermarket standalone device, like the Garmin Nuvi, suffer from this somewhat, but they seem to do a much better job all around than the built ins, but they pale in comparison to my phone in terms of everyday usability.
Of course, it's also always a good idea to have a reliable road atlas on hand - no matter the tech, sometimes it breaks!
