You're missing the #1 reason organized criminals prefer their own service. To have trusted staff who control everything — the servers, code development & deployment — whom can't be ordered by a court to shut off access to individuals at any time, or provide metadata, eavesdrop, etc.

The weakest link with legal services like Signal is that they can be compelled by law enforcement, the judicial system, and government... That's an enormous risk for any organized crime operation. Even a minimal amount of metadata collection can do a lot of damage, especially if it's analyzed over months/years, and especially when performed by an advanced persistent threat actor like a nation state.

The average criminal is no dumber or smarter than the average non-criminal. As such they’re every bit as subject to marketing ploys and mis/disinformation. so if their criminal buddies are using BaddieApp Pro, they probably will too. Or if they hear that Bill Gates is using the Signal app for mind control, there’s a good chance they’ll believe it.

I've definitely also thought about, if our government gets taken over by fascists, how do you organize a rebellion?

And yeah, Signal definitely has some weird fucking shit going on. As far as I'm aware, they don't allow you to use their centralized servers, if you don't use their provided build of the app. They don't seem to have a mechanism to enforce that, so you could still use a self-compiled build, but if all your friends are on a compromised client, you can't talk to anyone anyways.

Well, and then there's also the great stupidity that Signal requires a phone number. In my country, you can't sign up to a mobile phone plan without revealing your full identity. If the fascist government realizes that I'm part of the rebellion, they can make my phone number disappear in unfortunate circumstances.

So, yeah, I'd at least want to self-host the communication platform. I'd probably use an existing open-source solution, but would try to audit at least part of it...

Lets put it this way: There are criminals that use whatsup, twitter, and the like for communication. They don't really last for some reason. Then there are those who use a special, commercial system. They might fail if they fall for traps like EncroChat (or however that was spelled). Then there are those who try to set up their own system, but lack the capabilities and talent for that. And last but not least are those groups you have not heard from in the news. They do have proper infosec.

No, but by making their own they can ensure there isn't one. Or something like that.

Because they're not stupid and understand that government agencies could have a finger in the pie for any publicly available software in some way/shape/form? Paranoia keeps them in business longer.