The thing I hate about signal is the UI. Everything looks way too big on my device. WhatsApp, for example, holds 2 more chats, and the messages themselves are tidier.
This may seem like it's not a big deal, but UI is absolutely crucial on order to get people to actually use the app. I moved a few people to signal but they just hated the way it looks. "seems like an app for old people, font too big". I can see that. They moved back to insta/WhatsApp.
I think some small and easy UI changes could make the app much better: just give us a "compact" mode.
For me, I can see 7 chats on signal, 9 chats on WhatsApp. There are tons of wasted space on signal for me. It just looks bad to my eyes.