I think the benefits of a ChiGee / Carpuride / head units over a phone are:
- larger than some people's phones, but not that much
- touch is by pressure rather than capacitive, so glove presses work
- readable in sunlight*
- can cope with internal and external heat without the screen dimming
- not vibration-sensitive
- comes with mounting kit, which in Quad Lock terms is probably about £50-£120 saved
The other stuff (waterproof, 'rugged' - whatever that means) and definitely OS is no better than a phone.
And you still need to have your phone running in your pocket, so battery anxiety isn't completly eliminated - although with no screen on your phone battery should be good for say twice as long as if you were navigating with said phone.
*the preponderance of gambling shades over the screens (like all good grannies wear) suggests maybe they're not that good in sunlight though.