I'm the guy that wrote the blog article that got posted here. So, I'm dusting off my really old HN account, to step up and say that I am very sorry it felt depressing to you, because that was not my intent.
In fact, as I said in the post, I use Matrix, I love Matrix, I evangelize Matrix (especially to Discord people), it has made a lot of strides lately. I had a previous post on distributed offline-capable IMs at https://changelog.complete.org/archives/10205-roundup-of-sec... and I got so much feedback to "don't let people use Signal, switch to Matrix!" that I specifically was addressing Matrix as a Signal replacement in the post. (I regret it wasn't super clear that's what I was doing.)
I'm going to address your points in a sort of different order, starting with:
#5, encryption. The benefit to Signal here is that encryption is NOT optional. A user just knows everything is going to be encrypted. 1-to-1 chatting, group chats ("rooms" to Matrix), voice and video calling, attachments, EVERYTHING is E2E encryption and user error can't result in things being sent in plaintext.
Now as you say there are reasons that, say, #fx-desktop-community:mozilla.org with its 1100 users shouldn't be using E2EE. However, when talking about Matrix *as a Signal replacement*, the fact that E2E is only default but not mandatory for 1-to-1 chats, and optional but not default for multi-user chats (in Element), this makes it a lot easier for non-tech-savvy end users to goof and send messages in plaintext. A secure replacement for Signal needs to not have that option. Perhaps a "secure everything" mode in Element would help.
2. Fundamentally, there is a deeper issue here: for every Matrix user, there exists at least one, and possibly more, single points of failure. As far as I know, Synapse itself isn't clusterable, so for every Matrix homeserver, the failure of, say, a single CPU will render everyone on that server unable to communicate. I say "possibly more" because many people probably will run a singleton PostgreSQL instance also, though PostgreSQL can be clustered. Or even an upgrade to Synapse there would take people offline, etc.
For me to be able to recommend an IM to people, it must pass the "I can rely on this thing to get help if my car breaks down at night" test. Right now, Matrix doesn't give me that level of comfort. Yes, it's getting better. Yes, various hosting options exist. But still, if the homeserver you're using has a bad CPU or Synapse OOMs it or whatever, your messages aren't going through in a timely manner.
3. I am glad to hear that voice and video calling are getting so close. However, I want to still add that having two different VOIP systems -- one that can handle precisely 2 participants and another that can handle more -- is jarring for end users and admins alike. People are used to being able to tap "add participant" to their calls and this is a UX issue for people coming from other IM systems.
1. I'm glad to hear these RAM issues are getting better. I've spent a fair bit of time tuning that cache parameter, from its default of 0.5 down to 0.2 (which resulted in drastically unacceptable performance) up to the other recommendation of 2.0 (which still resulted in OOMs). I am in some large Debian, Firefox, and Matrix rooms, with over a thousand participants each -- though some of those, at least, are gated from IRC where such a scale is a non-issue.
I've been following Dendrite's "are we Synapse yet?" page with interest and excitement. I am looking forward to it being ready to use! But as I stated in my post, and as you are surely aware, it's not there yet, at least based on the "are we Synapse yet?" page.
4. It's not actually that simple. A person is most likely going to want at LEAST Synapse and Element Web. Most are probably also going to need coturn, Dimension, Jitsi, synapse-admin, and maybe an identity/directory server. The downloading of the software is the easy bit. The hard bit is getting all the bits talking to each other properly, with various JSON config files, keys, DNS entries, SSL certs, well-known files, ports, etc.
I set up my own Matrix server due to difficulties with the integrations at t2bot, some limitations in the IRC bridge I had to work around, etc.
Also to those wondering why Matrix and not XMPP: I used to run ejabberd and an XMPP service. XMPP has (or at least, HAD) a real issue when being used with multiple clients - delivering messages to the wrong place, not syncing history, etc. Matrix is far better with those things. Also Jingle in XMPP land barely ever worked, last I checked.
An aside: I try not to comment on non-federated corporate-control sites (of which HN is an example), but hopefully if you reply I'll get an email or something. You are also welcome to engage me on my blog or on Mastodon, where I first surfaced many of these ideas (and did tag the Matrix Mastodon account). Or email me.
Once again, thanks for what you do. Matrix is great for many use cases and will be great for more in the future. But I want to be clear-eyed about how it compares to Signal for the secure IM use case, today.