It's really hard to clean up media. You have to go into every single chat and from there go about deleting stuff. At least they finally added a "select all" option in there recently.
So the size of it just grows and grows and grows until it's using all the space on your phone. Not a good fit for non-technical types.
Secondly, no web view. There is the desktop app yes, which is flaky, slow and wants to update every day or two.
I just can't see average people putting up with those inconveniences and that's just a couple of them.
To be fair I've met plenty of non-techie types whose phones were "full" of stuff from WhatsApp or photos that had already been backed up, because the idea they could clear their local storage would never cross their minds. I've seen people buy new phones instead of clearing their cache.
The slightly longer version of the story is that my wife, travelling alone, had some trouble with an iPhone update (it hung for hours), and so she took it to the nearest Genius Bar; they eventually got the update to apply, but then did a factory reset “just to be safe”. Of course, everything except her Signal message history was restored from the automatic iCloud backups. She was devastated, and refuses to touch it now.
Please do not reply to say this was the fault of the Apple Store employee. It was, but at the same time, it also very much wasn't.
What is the alternative though? A private chat app, mobile + desktop, syncing, with enough ease of setup and use for normies to adopt? I don't see it.
iMessage, if you only use Apple devices or are willing/able to hack around the Apple-device requirement.
People behind Signal have a very corporate approach to their app where a permanent "no" doesn't exist when it comes to user choice - all what you have is "not now".
Then there's linking devices; it's not permanent but temporary and devices are removed automatically after 30 days. You can't even log into your account with tablet any more - that was replaced with linking. Cross-platform synchronization - didn't work for me at all despite being a loudly announced success.
This is at least an improvement over WhatsApp, which removes core functionality (e.g. creating groups) when this access is refused