I would never ever rely solely on a Smartphone for navigation
Navigation in the backcountry is so critical that I think relying on any sort of electronic device to keep you alive is foolish, but a tool can still be extremely useful without being something that you have to rely on (perhaps I am arguing semantics here). I have a dedicated GPS and a smartphone but the dedicated GPS is so clunky that I use the smartphone almost 100% of the time. A lot of smartphones don't work well as GPSs though: their GPS unit is terrible or they have bad battery life, or they aren't durable, or they run iOS and the available map software for iOS is kinda "meh".
I personally like OsmAnd a lot. It's easy to download maps, and they often have a fair bit of detail, like forest boundaries, cliffs, ridgelines (at least around here). It does eat up battery though, but the alternatives do to.