Back on topic ... as I understand it, DeLorme was acquired by Garmin. My experience with both companies was that DeLorme had the better maps for unpaved stuff, including obscure trails that had historically gone unrecognized by the Garmin maps.
In my most recent Garmin updates, I've noticed a number of unnamed roads that, when passed, turn out to be dirt or gravel paths. So my point is that with DeLorme's map base superimposed on the Garmin map base, the new Garmin GPS units seem to have even greater power.
We travel in remote rural areas with poor cell coverage quite a bit, and the iPhone 6+ has been quite a poor source for information for large segments of many trips. I would not trust the phone, even if it were weather proof.
And then finally, I can't say much about traffic updates that might be available on the phone. But on the Traffic versions of the Garmin, if you are in congested areas (where there is inevitably good cell coverage too), the Garmin Traffic reporting has improved with every iteration of updates. It still suffers lag, and it is probably no more than 65% reliable compared with a trucker on a CB radio traveling through the mess ... but it is a LOT better than when this feature came out.
And then there is the issue that the GPS does not require a monthly bill. And you can literally extend the useful life of GPS by adding an SD or micro SD card (typical cost from NYC discounters on the order of $10). And then there is the issue that I used to whine about ... the time it takes to update maps. But this has greatly reduced too, and it never takes more than an hour. Often it takes less than 15 minutes. This may have more to do with the data traffic on your network than it has to do with the GPS.
And a few have mentioned losing the phone, or weather issues. I have an HD ridin' friend who is pretty cheap when it comes to anything that's not chrome ... and he has ruined three cell phones on extended trips. The only GPS failures I can remember were early on when the Zumo 550 screens would fail. And Garmin stepped up and fixed them on warranty. That was my only experience with Garmin service, but it was first rate.
Well, I'm approaching the status of a post whore in this thread, so I'll boogie on down the line. Whipping Post is about to loop for the 110th play, and I don't want to miss a lick.