Cycling with a phone with the screen on and at full brightness (which is what you need on a sunny day), a phone will last a few hours at most. The magic of good GPSr units is that they use a transflective display, the sun is your 'backlight'. (And of course using something more akin to a microcontroller than a smartphone SoC.)

Also, in contrast to smartphones, these things are really rugged. Like last year I was cycling through the alps and accidentally dropped my GPSmap at ~30km/h (without a case or protection, who puts them on a GPSr?). It only has some scratches.

However, that combo is a battery hog. For some reason, OSMAnd drains a lot more battery when using it in guidance mode with Brouter, even though routing is much faster than with the built-in algorithm.

I heard that Locus maps has a much better brouter integration, though it is unfortunately closed-source: battery-efficient, automatic brouter detection, profile selection from within the app.

I also wish the brouter app would get a fresh coat of paint (a UI redesign), but that is secondary.

They have longer battery life, more predictable performance, and a wider range of operation parameters.

I’ll always prefer my specialized equipment for serious music listening, my ebook reader for longer reading and honestly pen and paper for serious note taking. They perform better and I get more performance with less effort.

I have an iPhone that I believe was damaged in this way.

So you’d probably want a purpose built bike computer, an older phone that you don’t care about the camera, or perhaps there are fancy vibration dampening mounts that could work.