Things that someone would only say if they have absolute confidence in their security, wouldnt be said on Imessage. It doesn't matter that I use fb messenger because I don't say anything that can be used against me; but without an alternative this leads to self censorship, projects not getting launched, development halted.

The only relevant question is, do you have absolute confidence in your security given your threat model? That's a yes or no question, there is no "well its better than imessages!