NetRunner88
As a matter of course, law enforcement will send signal a request for any data they have related to a phone number. Signal will then tell them that you use signal, when you downloaded it, and when you last used it.
Then a subpoena will be issued to you for access to your signal messages. Failure to comply would be illegal and if they can point to evidence of you having a signal account and that it was used recently the courts aren't going to rule in you favor.
Even with disappearing messages, they will still know who you contact via Signal when you comply.
SimpleX has none of that. It also allows you to rapidly and easily spin up separate identities for different use cases.
Signal is secure. It is also more private than SMS/Phone. It isn't truly private or anonymous. I recommend that everyone have a Signal account and route all text messages through it as a matter of course. There is zero downside of going so, it is a fairly seamless experience, and its fairly easy to get even tech illiterate people to switch.
If you are going to do things that a government might not like however, or anything you want to be positive can't be tied to your real (or any other) identity then use SimpleX.