I use both my phone and built in nav system in my truck. No experience with garmin/tom tom/etc.
Many of the objections to phones here are no longer valid or paritally valid. GPS works wtihout cell coverage but you needed to route while you have coverage or have downloaded the maps. No issues with phone calls, etc.
One issue I have that has been mentioned and can be a deal breaker is my iPhone 6 overheats when I keep the map on. It did it to me last Saturday and it was a mild day but phone was in sun combined with google maps. Fine for city but not long term. It does zap the battery too but you can charge it easily in the car.
What I like is traffic info so I will often use phone and truck nav system to see traffic if I expect it to be congested.