I think I must be missing some points with dedicated GPS units.
I totally agree with that they are robust, designed for the use etc. But the screens are small, and they have only one use. I presume that like any electronic item they can fail?
1. Maps. My phone has the 1:25 maps of the whole of the north of England. A second cheap phone can have the same. (With or without GPS). Though in the event of catastrophe I can always just go back.
2. Screen size. I understand that the typical size is 2.5". I started with that size screen years ago, its ok, but now my screen is twice the size. Which means that even without my glasses I can still see enough.
3. What information do I want? Grid ref I have but don't need. I can see where I am at a glance and my direction of travel.
4. Robustness. Dedicated unit wins, though not always. Plus I can add robustness for a small cost. (Rhino has the perfect one)
5. I can call for help, call home, get weather info, get advice, etc etc. I also have a camera and many other things.
6. My GPS is free. and will act as a satnav in the car, and play music in the car. I can watch BBC news in the car while waiting. Or films or whatever.
Also, there are now very cheap tablet devices that also have GPS built in. For anything between £50 and £200 you can have a 7" screen and backup. Typical weight 160 grams.
Sorry, I yabber on after taking pain meds.