Three answers:
No - I use my phone for road navigation, including anywhere where there isn't a cell signal. Nearly any navigation program allows you to download the maps to your phone when on wifi, and then works just fine regardless of cell signal (except for traffic updates, which to be honest, hasn't been an issue in those areas). Multiple trips, multiple locations with no cell service, and no issues. (I realize this a is a problem for anyone who believes that using your cell based GPS will give someone your data location. Reality is just using your cell at all has already done that...)
We had this debate on another board I frequent - and folks there argued about not liking the small screen on the phone (which is a non-issue there, as the maps come up on the 8" screen in the vehicle with android auto or apple car play), the non-cell areas (works fine with downloaded maps), and just general I like the truck based system better. Pretty expensive option at nearly 800 bucks for on board navigation, and I pointed out my truck even has a CD player that has never been used, but the automakers keep them for those who resist change...
Yes - I use a Garmin handheld unit for fishing. It has a lot of waypoints stored in it, and means I'm not fumbling with my phone in the boat all the time. (The the navionics app works pretty slick too).
Yes - I use a RTK GPS unit at work - which is survey grade and gets me within extremely low tolerances (and allows for even vertical elevations within 0.1 feet). These are expensive, and get upgraded every few years...