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.