That used to be true, but now the desktop client has almost all the feature of the mobile clients. Which feature do you miss there?
> Signal clearly has more users (while being worse on privacy)
I assume you're talking about the phone number requirement? This is fair criticism, but what about the rest? Signal leaks way less metadata than Wire, which is more similar to WhatsApp in that regard.
See also this comment: https://news.ycombinator.com/item?id=14069674
Some things might have changed, but it's not clear to which extent.
Since centralized services are currently also the convenient ones and neither Wire nor Signal show any sign of wanting to use a decentralized protocol, those that can't be bothered to use inconvenient services have to trust that their centralized service is ethical about what they do with your data.
The privacy differences are very minimal by comparison when they're all missing one basic feature or other like message editing (Signal), a desktop client (Threema[2]), cross-chat searching (Wire), searching a chat at all (Element), any usable encryption (Telegram), etc. If you're going to use an end-to-end encrypted messenger where the server can't read any contents, the privacy differences are just not that large when you trust them all equally. The only thing that you can objectively compare is what the client sends, since that's something they can minimize and you can actually check.
[1] "clients derive a 96-bit delivery token from their profile key and register it with the service. The service requires clients to prove knowledge of the delivery token for a user in order to transmit “sealed sender” messages to that user". It's a bit opaque so in regular English: my Signal client sends something like deliveryKey = H(profileKey, currentTime) to Signal, where my profileKey is something only my contacts and I know. Great, so my contacts only specify the deliveryKey and not who's sending, so you send anonymously! But wait, those contacts connect to Signal with an IP address and are doing other things like updating their profile or registering their delivery keys for their account from that same IP address. 1+1=2 and you know who is sending messages to whom.
[2] They have it, but your phone plays Chinese Whispers with your computer and every time your laptop or phone reconnects to wifi/mobile data you need to open the app on your phone and navigate two menus to re-enable it.
With Matrix you still end up trusting the server sysadmin (both in terms of ethics and technical abilities), it's not like it solves the problem for mass communication where at least one user has to agree on a 3rd-party instance.
> those contacts connect to Signal with an IP address and are doing other things like updating their profile or registering their delivery keys for their account from that same IP address.
Correct me if I'm wrong, but I think that happens within SGX. This in itself is its own can of worms, but assuming it's secure, I don't think you can do this association IP / account that easily. At least it seems so easy to work around that I would expect it to be like this (given that they already use SGX for other things). Now of course SGX is not secure, but it still makes the attacks more expensive and complex than they would be otherwise. If you have access to a Wire server, it would take you minutes to get all the group memberships of one user. If you have access to a Signal server, you'd need to log connections over some time, and do some statistical analysis to extract useful information. Not impossible, but far more costly and less reliable.
In terms of privacy Wire might be better with respect to the phone number requirement, but otherwise Signal has an edge.
The IP address is also something you can solve independently (VPN / Tor), so it makes sense not to solve it within Signal. But it would be nice to have some integration with Tor eventually.
It likely doesn't really add much security. Anyone capable of running processes on the chip hosting the SGX enclave can probably run a side-channel attack to recover the necessary keys. I was very disappointed that Signal took that approach.
Features are available and work everywhere (unlike Signal which has a dumbed-down desktop client and no web client at all), it does everything you generally need and the search is actually superb (better than Telegram even, since tg only does word matching and Wire can do symbol and arbitrary string matching and is also very fast) although limited to one chat so you need to know which chat contains what you're looking for. Meanwhile Element (Matrix) goes "can't search encrypted chats"... useless if you want to communicate something non-ephemeral, you'd need to switch to pgp-encrypted email and all its problems or another chat service.
Compared to all the alternatives, Wire with all its faults is the best encrypted messenger. I would recommend it to everyone aside from the network effect: Signal clearly has more users (while being worse on features and privacy). Because it's useless to be alone on a messenger and because it's still a step forwards from the status quo, most of the time I end up recommending subpar solutions.