There can be good reasons to use a smartphone as the computer. Many folks just buy a cheap used phone, no SIM card and just load RideWithGPS or Strava or something similar. Most reported issues are battery life can be poor, you need to finagle the phone to have the display on all the time, screen can be difficult to read in bright sunlight, you might need a waterproof case (phone model dependent), phones are typically much larger sitting on a h-bar, not everybody wants that huge phone out there.
Lot of folks say why bother and just get a dedicated cycling GPS device.