The 'headphone hearing protection' would make the volume too quiet to hear over the engine any time you jiggled the headphone jack. Required a screen interaction to fix.
Of course if you didn't hear the audio prompt or couldn't make it out precisely, you're on your own - no way to check it or get it to repeat if the phone's in your pocket.
Cheap rain-proof phone holders have a zip around the outside - so you can connect wired headphones or a charger - but not both, as they poke out different ends of the phone.
Wired headphones linking you to something linked to your bike make getting on and off complicated.
Motorbikes don't have standard cigarette lighters to plug a standard charger into. You'd have to mess with the bike's wiring to add a phone charger, so I went without.
The battery wasn't good enough to navigate a journey that was 60 minutes each way. Google Maps didn't offer any options like automatically turning off the screen on long straight roads.
Once your phone's in a waterproof holder and you're wearing motorcycling gloves, your touchscreen won't work. So you can't quickly cancel navigation to save battery (or dismiss the hearing protection warning) while stopped at some street lights.
All things considered, it was pretty clear the navigation experience wasn't designed for motorcycling.