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.
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.