I barely ever use my Garmin for routing anyway, so the turn by turn stuff isn't important to me.
I like to convert routes I get from elsewhere to tracks, make them visible on screen and just kind of follow them. What I do is set the track I want to follow as red, and set the track I make to black and say to myself "I see a red track and I want it painted black". That will help you find the entrance into the bush, and you don't really need the GPS when you are in the bush unless you come to a crossroads and need to figure out which way to go. The other main use for my garmin is recording the track so later I can track back or I can share the route with others.
I do not use my cellphone on my dirtbike as even the tough Garmin's get rattled to pieces and I haven't found a cellphone holder yet that holds on to a phone tight enough for offroad use (X-grips certainly don't, as I found out last weekend).
I occasionally use my phone or 8" tablet on my vstrom, but even then there are upsides and downsides to it.
The upsides to using my tablet on the bike are I have a huge view of the map in locus pro, so I can see which roads don't continue, I can load terrain maps to see which route might have better scenery, and I can plug in headphones to listen to music from the tablet.
The downsides (to me) to using my tablet is that the x-grip is not secure enough and I've had the tablet bounce out a few times. The tablet is not waterproof so I need to cover it. The screen doesn't work well with gloves, and products like anyglove or thumbdogs don't work well, so every time I want to pan the map over I need to remove a glove. The glare is terrible -- much worse than the Garmin, so trying to see the tablet in bright light sucks. Finally... longevity... I don't know how long my tablet will last due to the vibrations from the road and the sun beating down on it. My Garmin Oregon 450 is falling to pieces after 40000km over two seasons, and it is tough and waterproof.