Anyone who says Signal isn't private doesn't understand anything about cryptography. Metadata is another matter. Signal is private but not anonymous, Simplex is private and anonymous (or tries to be anonymous because data correlation attacks can be carried out). Although advances have been made in Signal, information can still be gleaned from metadata, such as the last time you were connected or that the server knows the recipient of the message, i.e., who you are sending it to. That said, Signal is the gold standard for privacy and is more proven than Simplex. In fact, Signal has been a pioneer in many ways. Both Simplex and Signal are two tools you need and can have different use cases.