What's hopeful in all this is that Signal is, slowly, catching up. Slack can roll out new features just by assigning a couple developers to it, and Signal has to coordinate new cryptographic research --- not just new cryptographic research, but research that produces something deployable at scale within the resources of a project like Signal! --- so Slack (and Wire and Keybase) are at a permanent advantage here.

But over time, Signal gets more and more usable without having to consider tradeoffs.

Which is number one reason why I'm not even considering it

"some people don't like this because they want to use multiple ephemeral usernames"

That's not my reason: I don't want people I don't know to get my phone number through other people I know and trust, but are used to share everything online. Of course that would be possible without any social application as well, though using one makes it much more natural.

Then this one from their site: "Multiple mobile devices and Android tablets are not currently supported"

Triple facepalm here: this makes it even worse than Whatsapp I use (read: am forced to use) on an old tablet. Whatsapp sucks badly just at everything (didn't I write I'm forced to use it?) but at least I can read what I write.

Downvotes welcome, though advice on secure+open alternatives that don't assume I have a smartphone (I haven't one and don't plan to) would be more informative.

Here is a story what has happened to Doubi (SSr developer.) He was a very well aware of anonymity risks, and he evaded police for years on end. China literally tried to do geolocate him by turning off the internet in entire cities, but to no result — he caught on to that, and started randomising his release timing, and avoiding releasing "hotfixes". So, the entire Chinese police and MSS been looking him for 4-5 years.

What has happened? A few month before his arrest, he registered a Twitter handle with a throwaway SIM card. Those are being usually sold by "grannies" in Chinese 2nd tier cities who peddle things like fake tax receipts, anonymous train tickets and such.

China either hacked Twitter, or had somebody bribed there, and they got the number. They then tracked down the granny who sold him the SIM card, and went on and checking every person door to door in that small town. Then, they found him.

He got 5 years prison, and 4 years of laogai (gulag)

This is wrong. You could always have multiple desktop clients. You can also add iPads as linked clients now. Personally, I have two desktops and an iPad linked to Signal.

WhatsApp doesn’t support linked devices at all, the web client connects through your phone. Signals linked devices function independently, you can power off your phone and they’ll still work.

You are right about the mobile client, but that's not true of desktop. I have Signal installed and setup on every desktop/laptop that I use without any issue.