Only the potential to run down the battery on the phone, which makes it unavailable for other uses, like a phone call. I suspect a dedicated cycle computer will have at least 12+ run time, don't think you can get that with screen on all the time on an iPhone. Maybe think of bringing a battery stick along just in case. Using a smartphone can work, there are many who buy a cheap waterproof Android and run RWGPS or Strava or Kamoots or something and are perfectly fine with it. I'm watching a YT series of a couple touring across the US, currently in Ohio, the guy is using his cell phone with Kamoots, seems to work mostly OK. Using a cheap used phone means you don't worry about failure in a crash or battery life on your expensive and primary phone. Your call as well of the screen is bright enough in the sun, which can be an issue.