Having a messaging app that works well and is secure on a bunch of supported platforms is hard. Having a messaging app that works well and is secure on random exotic OSs is even harder. And having a messaging app that works well and is secure with third party software and federation has not yet proven to be feasible AFAIK.

Signal seems to have choosen challenge #1 and making a good job at it. I'm not sure they should be blamed for not choosing challenge #2 and even less challenge #3.

Now what I really don't understand is why they made signal open source. They clearly don't want third party redistribution/packaging of their software, so they should really have chosen a "source available" licence for code audit, which would have better set expectations