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:

1. 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)
2. With these apps communication is encrypted by default unlike cell tower communication
3. The range is limited. If police listens over Bluetooth they'll be able to track less people down