Compliant USB-C cables might not always work at full speed (or at all, on some devices that need more power to operate), but they will never be a safety hazard. The only possible safety hazard with USB-C charging is a noncompliant cable with an E-markers that explicitly claims to support 5A charging, while the wires are sized for lower current (which would be a conscious decision made by the manufacturer to build a serious fire hazard.)
(Of course, cables and connectors can be defective, and there's always the risk of this. But the USB-IF has spent a lot of time making sure compliant devices can never be configured in a dangerous way. It's up to the manufacturers to only build devices and cables that comply with the specifications. Unfortunately, this is less common than you would think. Amazon happily sells horribly non-compliant dongles and cables that easily have the potential to damage electronics or start fires.)
(Of course, cables and connectors can be defective, and there's always the risk of this. But the USB-IF has spent a lot of time making sure compliant devices can never be configured in a dangerous way. It's up to the manufacturers to only build devices and cables that comply with the specifications. Unfortunately, this is less common than you would think. Amazon happily sells horribly non-compliant dongles and cables that easily have the potential to damage electronics or start fires.)