1. Tracking via bluetooth, wifi, cell signals, nfc, etc. Does one trust airplane mode?
2. Seizure of the device if one is arrested. There is legal debate about what methods law enforcement can use to get into the phone. One is exposing both whatever pictures and video was made at the protest but everything else going on in one's life too.
3. If one has a unique case or model, one can be doxxed.

Action cameras are cheap, durable, and many come without any radios that can be used to track someone. They all look the same. Using a brand new sd card means that the only data on there is the pictures/video taken at the protest. The major downside is that if they are seized, they are an open book for law enforcement since they are unencrypted. If the sd card is taken or destroyed then one loses any evidence along with it.