For example: most people want to be able to back up and/or transfer messages to a new device (which can happen every few years) or sync to another device such as their desktop so that (gasp) they can see message history when corresponding with someone.
Probably half a dozen times in the last 5-6 years I have lost all my Signal messages.
Most recently, I did a full, encrypted backup of my iPhone to my Mac (which has FDE via the T2 chip) prior to doing a device wipe because it was starting to feel sluggish. All the devices involved have the best security of any handheld/portable computer commonly available and the data was DOUBLE encrypted on my Mac. Every other app's settings restored fine....but not Signal. They exclude their data store from the backups so I lost numerous messages, photos, etc from a couple of friends. Those messages are gone forever.
I can't use Signal for any transactions with other people, even something like a simple Craigslist deal, because there's no way to export messages to provide them as evidence, other than to take a screenshot. So Signal is also useless to lawyers and business executives, too.
When you ask them why they don't support the various things like exports and syncing, there's a lot of general, paranoid hand-waving about how if you're a reporter or human rights worker in a third world country those features could be abused...like said reporter or human rights worker wouldn't simply be tortured until they unlocked their device...also ignoring that it should be perfectly possible to allow the user to choose (gasp!) whether they want to enable such features. Ie: let them decide for themselves what their risk profile is.
You know why their claims are bullshit? Because they've never implemented a duress password that unlocks the app but only shows messages from, say, friends but hides messages and contacts you don't want people to see.
When I mention syncing people start shrieking about cloud storage this and that...which you don't need to sync two devices, as demonstrated by syncthing, magic wormhole, and a slew of other programs that are device-to-device with, at most, a zero-knowledge coordinating proxy, which Signal already has.
So can I please be allowed to back up / export my messages and call history? Can I please see my message and call history in the desktop app? Would you please support backing up my message history to another device with the best fucking device security in the world, that is double-encrypted?