This isn't really the primary issue with Signal, as the protocol is pretty well audited, tested, and cryptanalyzed.
THE issue with Signal is using real phone numbers as identifiers.
I know this point has been made repeatedly, but it's important to beat the drum, since it's recommended for journalists and dissidents, and that this "feature" will quite possibly get you killed if you're a dissident in a totalitarian system with a very simple two-part process:
1. All it takes is ONE of your fellow dissidents to turn snitch (or be tortured, or have their phone cracked) and disclose all their contacts.
2. Burners are NOT anonymous. Everything from location analysis to CCTV footage from the physical place you bought the SIM will give you away.
Signal is an obvious honeypot. Moxie's total refusal to even allow the option to use throwaway identifiers while burning precious development time on silly things like emoji support makes their purpose very evident.
Sure, use Signal to send cat pics. But stay far, far, far away if your life or freedom is at risk.