I found this interesting 40 hours case study of Signal, by Built For Mars. Two key takeaways seem to be that prompts must be contextual. And, being a privacy-first product, it must be even more important to ask for permissions only when necessary. It’s actually better to read the post below, to get a visual overview of where Signal can improve on. A related Hacker News thread is open too.
I don’t remember how the competitors like Telegram and WhatsApp fare, but I am keen on giving it a look myself. Maybe sometime in the future.