You can't delete Signal contacts, like, ever. My signal address book and synchronized chats are flooded with plain numbers, or self-titled user names (which also invites for identity fraud...), because I deleted those contacts in my phone (which ofc I need to do, because it's used for privacy settings in other apps...). That's a problem for many years now, the github issues get closed by maintenance bots.
I am a somewhat tech-savvy person, but even for me the user experience leaves me annoyed and uncertain about some basic functionality you expect from messaging apps these days. I stopped recommending it to people some time ago. Don't want to blow my chances to migrate people to something fundamentally better.
Comically, every platform has different issues with "deleted" meta-info popping up again. Like "deleted" groups, "deleted" contacts, "deleted" chats... meta links I can't get rid off whatever I do. The worst is legacy groups I left, "deleted", can't even use or join anymore, but also can't get rid of. They reappear every time I refresh the iPad app. Mind you, the group members are still visible, so I am really happy whistle-blowing, terrorism or crime isn't my profession.
This made me really distrusting of Signal, when years old meta data popped up again. Like, have they lost some pointers or something? Can I even expect the data to be gone, when I delete my account for good? Doesn't feel like privacy at all...
The UI looks like iMessage. So it doesn't seem it should be any issue there.
To me signal is the best UI, and WhatsApp looks like a child designed it. Especially with the horrid default colors and backgrounds.
But I don't really see a difference in UIs in most chat programs. There are only two types of UIs. Point-to-point (signal, WhatsApp, imessage, etc) and room based (irc, matrix, slack).
Their all so similar, I don't understand what is better or worse in Signal compared to WhatsApp.
On the other hand, both of these things are essentially limitations imposed by SMS, at least when one end of the conversation is not an iPhone. Both can be fixed, at a small cost in complexity, and if they did that, I think it would hit a good balance between features, usability, and architectural complexity.
UI in general feels pretty much the same, but things like actual desktop apps with decent UX make a massive difference.