Are you looking for arguments against Signal?
I think the biggest one everyone will find and is very difficult to overcome is “My friends aren’t there”.
Other arguments like “it’s centralized”, “It’s in the US”, “It doesn’t have feature X”, “The client app doesn’t have fancy stickers” are workable, have explanations or are a matter of time.
Thos saying “It needs to support SMS”, are americans that weren’t really using signal, but a glorified text app.
They treat this as if e2ee was the privacy grail but it’s only marketing to fool people believing they’re protected.
The actual contents of the messages aren’t as important for privacy. It’s the Metadata and a ton of other measures rhay signal implements in their family of protocols.
Talking about e2ee and call it private shows ignorance in what privacy entails.