I found my gopro coords (based on mapillary uploads, I’m too lazy to pull out gpx) to be somewhat more accurate then the phones. Also tend to find iOS post processing the coords too heavily, especially in sharp turns. Plus, as you mention, signal interference is worse with phones. All of my phones are pretty bad with gps accuracy, I could only go sub-5m with an external receiver.
But then when you get multiple gopros set up, you notice that they drift in relation to each other as well. too bad