The phones will never be as good a decent GPS. Well at least with current technology, in the future, maybe. But if you don't really need a good GPS then they aren't bad. I use my GPS for a lot of stuff and am just used to using it instead so that is my preference. I actually use both the phone and GPS at times, Google Earth is nice to have available.
I see 2 downsides to using the phone. Primarily battery life. You'll suck the phone dead in a couple of hours if using the GPS a lot and then you'll not have a usable phone in an emergency. It is possible to use re-chargers in the field, but not nearly as easy as simply swapping out AA batteries in my GPS. I can keep my GPS on non-stop during daylight hours for several days on a backpack trip if needed with just a handful of spare batteries and keep the phone off unless needed.
The 2nd problem is reception in thick woods or steep mountains. A quality GPS, especially with an external antenna is much better at this.
When using your iPhone as a GPS, you do not need cellular or internet coverage. Pre downloaded maps work just fine without them.
Also, I have used my iPhone SE on 12 day backpack hunts, sustaining the battery with nothing but a lightweight external battery pack and solar charger. On airplane mode, these phone batteries will last a long time.
I carry a Garmin in case SHTF, but I used it a grand total of 2 minutes in my 2016 hunting season.
I personally use TopoMaps on my phone, but there are a multitude of great map applications.
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