What is unsafe about cell communication is that there is virtually no way to prevent being tracked. Bluetooth is not necessarily more secure but has several advantages:
- Its Mac address can be easily changed. With a fake Mac address it's more difficult to track someone down, albeit it needs some "technical skills" (going in the developer settings)
- With these apps communication is encrypted by default unlike cell tower communication
- The range is limited. If police listens over Bluetooth they'll be able to track less people down