Mainly that Google essentially owns it now. It’s got cool features, but it’s reliant on too much bullshit. That’s extra true once you factor in that there are services that do the same things, better. Even signal is better, feature wise.
If we’re supposed to have some third party company with their nose in our communications, there’s better options than Google. It isn’t like they’ve gotten perfect reliability down, they have plenty of lost messages, outages, etc. So, why the fuck bother?
It does suck that Google gave themselves a monopoly on it on Android. But I personally find it much better than Signal as far as features go. I think Signal tries too hard to be secure at the expense of features. My old phone broke and I lost all my messages. Nothing I can do to get them back. Luckily I only used it when my parents were out of the country, but if I had old messages and photos from a friend that passed, I would have been heartbroken. Signal was basically dead to me after I realized that sometimes I can lose messages.
I still have my texts from when my wife and I started dating almost 15 years ago.