People say this over and over "depends on your threat model" and yet people seem to have a hard time understanding that. Your threat model is "who is your adversary and what he is willing/able to do". Your security goal is what do you want to keep from your adversary.

As others said, if you are an activist or sth important, perhaps you might want to build a working knowledge of cryptography yourself. If you just want META not being able to see your NSFW chat with your romantic partner Signal might be more than enough. In fact, people way more relevant than me also suggest that Signal is good even for bounty hunter vulnerability reporting.

Having said that, what bugs me most is that people think the instant messaging format as suitable for everything: activism, jobs, crimes, broadcasting 1970's prog rock for extraterestrials , whatever lmao. Do you really want to use your phone for all that? Like, just carrying the phone around in the first place nullifies your other precautions, for all advanced threat models beyond privacy of non-critical social messaging.

Persistent/resourceful adversaries can eventually get to you, using a set of penetration and intelligence techniques, which means, if you are involved, the convenience of messaging your partners in crime from the phone in your pocket while waiting for a bus is a convenience you probably can't afford.