In my experience, the one advantage of dedicated bike computers is their ruggedness. A phone is more likely to be damaged (although it never happened to me), and driving rain will make it next to impossible to interact with a phone (the tactile screen confuses raindrops with gestures).

Other than that, phones are superior on all features.

It is fairly easy to keep a phone fully charged. Turn the screen off when you don't need turn by turn directions. A battery pack may also be a useful addition to your kit.

As others have indicated, phones can be used offline. I use an old S5, without SIM. Occasional WiFi access may be useful, but rarely necessary.