I find it particularly annoying that it inherits almost all of the worst features of WhatsApp which people are supposedly fleeing, such as being completely centralized and tied to your phone number.
Signal on the other hand is a nonprofit run by trusted people like Moxie Marlinspike, it's accessible to casual (non-IT) users as well (unlike XMPP) and it's fully open-source. It also minimizes metadata, like with the Sealed Sender functionality.
Just out of curiosity and not challenging: what makes Moxie trustworthy?
Signal also has an excellent user experience (much better than I was expecting) and most of the privacy concerns are alleviated. Decentralised would be better, but I have not found Matrix to be a particularly smooth experience.
Signal being centralized and tied to a phone number are design choices to make things easier for users. That's a far cry from the presumed reasons WhatsApp did things. The only reason that Signal has gained what traction it has is because of those features.
My grandmother can easily use WhatsApp or Signal.