If you are using your phone as a GPS then its coverage as a GPS is the same as a dedicated GPS. Of course, your maps like on your dedicated GPS should be offline maps. For example, Locus provides offline maps (Android users).
I assume that there would be apps for the iPhone that also allow for maps to be offline (downloaded).
I have used phones in the past but have trashed two phones due to vibration damage, and that was using vibration isolation mounts.
As mentioned in an earlier post, some phones have higher vibration ratings than others, while some are even categorized as "rugged" phones, yet still seem to fail motorcycle use for many users as noted on several forums, while numbers of others report no problems.
Regardless, my reasons for moving to dedicated GPS units, specifically motorcycle designated GPS units is for various personal preferences.
For one, even though I regularly back up my phone, I am no longer risking my phones to unnecessary risk due to the data and info I have on them, especially given the escalating prices of phones. My Zumo XT on sale cost a third of my phone price.
Another is some of the features I've found to be indispensable to me on motorcycle specific GPSs, most notable being the Zumo XT's glove friendly screen. No special fingtip gloves needed. Even with my thick, insulated winter gloves, my touches and selections on the screen are immediate and responsive. I have yet to find or even hear of a phone that even comes close to working this well with any type or material of glove.
Foregoing additional reasons, one of the most important to me, due to my riding and the areas I explore in the US and Canada, is the quality and type of GPS receivers and chips on phones compared to dedicated GPSs.
Because of where I work, I am exposed to specialists and professionals in the field of avionics and navigation, who are very familiar with the GPS receivers in phones and dedicated units and they will be the first to explain to you the lower quality, accuracy and capabilities of phone GPS units compared to quality stand alone GPS. Not being the phones fault, but due to size and antenna limitations of phones. They're primary function being a phone after all. No phone GPS currently has the capability to lock as many satellites nor has the signal strength of a quality GPS unit, nor the available dedicated processing power for dedicated GPS operation.
Some say buy a cheap phone or better yet, buy a cheap "rugged" phone for motorcycle use, but as explained to me, those cheaper phones and Chinese rugged phones also have cheaper GPS receivers and chips in them. Yet many seem to have OK experiences with them. But for the price, I can definitely find a GPS in a similar price range.
For general, fairly unobstructed sky navigation use, phones work fine for me, better for others. But in several instances where I've ridden in heavily treed and mountainous areas using my phone and stand alone GPS, I've often lost a signal or had ridiculously long refresh times or recalculating times on my phone whereas my GPS never missed a beat.
As in all things of this nature, YMMV, but from my experience, type of riding and where I ride, I much prefer and trust a dedicated GPS unit over a phone, regardless of app or phone. Others have had better phone GPS experiences than I have, but personally I won't depend on them anymore nor expose my phone to the potential damage and elements that my GPS is actually designed to to take.