Similar to another discussion I'm on in this same thread, Signal is simply non viable for my use case at this time. I have the app on my iPhone but I have no conversations in it. For background, my primary IM service is Facebook Messenger, followed by Telegram for some chats, followed by Discord for larger groups around games.
Signal doesn't have a native (or even near-native) desktop app for macOS and Windows nor does it support my iPad. For an app I rely on as much as my messaging software it must be available and high quality on every platform I commonly use. Signal's desktop app is poor compared to the extremely polished Telegram macOS app and the nearly as well made Telegram Windows/Linux/macOS app. Admittedly Messenger doesn't have a very good Windows app but in my opinion it is still better than Signal's, while it completely lacks a macOS app which is a bummer.
The Signal iOS app has no way for me to back up my messages, and I understand they are not backed up on Signal's servers. This means if I lose my phone I will lose all my messages. On IM services such as Facebook Messenger and Telegram this is a non-issue for non-E2E chats. I realize this is considered a feature for many, but for me it is a strong anti-feature.
And finally yet most importantly - practically no one I know uses Signal. Even the 5 or 6 contacts it found haven't even set profile photos nor did they reply to a test message I sent - it's likely they've uninstalled it from their phone. Everyone I routinely interact with save maybe two people are on Facebook Messenger or at least visit Facebook.com eventually.
If you do ever find a great chat app, let me know as it sounds like our considerations are similar.
I have a few people I chat with that I'd prefer the conversations not be generally available. There's little friction to installing whatever apps we need to get that done.
We want confidentiality, but not inability to do what we want with our own data - as you say, the inability to sync or backup messages is a strong anti-feature. I'm regularly screwing around with software on my phone and don't want to lose all my messages every time I screw around a little too far.
It's okay to tie accounts to phone numbers, but the app shouldn't require (or be an asshole about not allowing) access to things like phone books, SMS, etc. (This is what initially turned me off of Signal.)
Security issues and all, my figuring with Telegram is that at least if anyone it's the Russian government collecting information on me this time instead of the US, China, Google, etc. I'm uninteresting enough that simply spreading it around a little should help in avoiding anyone learning too much about me.
Signal doesn't have a native (or even near-native) desktop app for macOS and Windows nor does it support my iPad. For an app I rely on as much as my messaging software it must be available and high quality on every platform I commonly use. Signal's desktop app is poor compared to the extremely polished Telegram macOS app and the nearly as well made Telegram Windows/Linux/macOS app. Admittedly Messenger doesn't have a very good Windows app but in my opinion it is still better than Signal's, while it completely lacks a macOS app which is a bummer.
The Signal iOS app has no way for me to back up my messages, and I understand they are not backed up on Signal's servers. This means if I lose my phone I will lose all my messages. On IM services such as Facebook Messenger and Telegram this is a non-issue for non-E2E chats. I realize this is considered a feature for many, but for me it is a strong anti-feature.
And finally yet most importantly - practically no one I know uses Signal. Even the 5 or 6 contacts it found haven't even set profile photos nor did they reply to a test message I sent - it's likely they've uninstalled it from their phone. Everyone I routinely interact with save maybe two people are on Facebook Messenger or at least visit Facebook.com eventually.
edit: grammar and expanded responses