>>pera+(OP)
The biggest drawback with Signal for protesters is that it exposes the user's phone number to everyone else in groups (just like WhatsApp does). There is no way to even hide the fact that you have an account on Signal. I can add phone numbers by enumeration into my contacts and Signal will show who among my contacts is on it. If the authorities don't use tactics like they did in Hong Kong, the protesters may be safe from being spied on (or worse).