Phone batteries aren't up to extended use with screen at sufficient brightness to be seen in direct sunlight whilst also running GPS/glonass etc. and turn-by-turn. Fine if you don't mind stashing a battery pack somewhere and having cables all over the place but a bit of a faff otherwise.
There are other good, reliable and more affordable bike GPS computers available besides Garmin. Wahoo, while generally reliable and user friendly, still ain't cheap. The Lezyne units are excellent for a fraction the cost of a Garmin and probably a fair bit less than a cheap smart phone, battery pack and associated mounting brackets. That said, if you've already got the phone and accessories and are happy with the features and performance there's no reason not to stick with it.