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.
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.