Nothing wrong with Signal except sharing metadata with them.
Discussion
"Signal also uses our metadata encryption technology to protect intimate information about who is communicating with whom—we don't know who is sending you messages, and we don't have access to your address book or profile information." Nov 16, 2023
Even if their backend was opensource too
I don't think there's anyway to ensure that is actually what you are connecting to.
Nice that they say so though.