The most common reasons are battery life of your phone and lack of ant+. There are some phones with ant+ now but not iphoiP as far as I know. I used to have a Garmin but now I'm using my Android phone with a mount and I have no complaint . Bike computer apps use GPS so they can drain battery so if you are the type that is going on 4-6+ hour rides that may be a concern . My rides are usually under 2 hrs and my phone gets great battery life so I don't see that as an issue for myself anymore. There are plenty of Bluetooth accessories so that ant+ isn't an issue. I use Bluetooth cadence, speed and heart rate with no problem. Some people claim ant+ is more reliable and less prone to interference but I think the latest Bluetooth protocol gets rid of most of those issues .