Whether it's a phone or a dedicated unit, for cycling you are best off figuring your own route out beforehand than using turn-by-turn navigation. It will make for a much more pleasurable experience. A bike specific routing algorithm that allows trails will likely take you down some muddy hell of a footpath, and a car specific one down some hideous dual carriageway or even a motorway, and avoid a handy alleyway cut-through. Much better to figure it out yourself first on a map, program that into the device and ask the device to follow that, or navigate by following a line by eye.
That's all IMO, btw.
I never think twice about asking my car satnav to navigate me somewhere, but I'm far more wary when on the bike as I'm more picky about the route, preferring nice lanes, avoiding monster junctions. Of course if your requirement is different -say you are making a load of deliveries or something, then you'll come to different conclusions.