The specification as-written is really, really bad (though I don't think this is a fault of the protocol),
Synapse is kind of...and work on Dendrite seems to have mostly stalled,
the billion people signed up on matrix.org has allowed for a sort of de facto centralization (not a huge problem, though in practice it sort of is, given how much the server buckles under the weight of all of the users; might be solved if it's ever moved onto a more performant implementation?),
the lack of encryption by default is iffy (though done for understandable reasons).
To lend credence to the specification being bad:
"I tried writing a matrix homeserver but I couldn't figure out how the state resolution process works" said by the author of Audacious/one of the authors of Pleroma/much more, so certainly a person who should be able to understand something that simple:
https://socially.whimsic.al/notice/9oxtjVqQw8gRS6QDKa
I know multiple people who've tried to implement the spec but who have gotten stuck on things that should be simple but are unclear if going by the spec.
> The specification as-written is really, really bad
We get a pretty wide range of feedback on the spec fwiw: some people seem to really like it. Others say that it’s “really, really bad” which doesn’t exactly give us much to go on...
> Synapse is kind of... and Dendrite is stalled
We have no choice but improve Synapse currently, and while it is still quite a resource hog it’s improved by at least 3-5x over the last year. Dendrite instead has become more of an R&D project for future homeserver shapes, but it’s not entirely stalled.
> The matrix.org server causes de-facto centralisation
The server has been less than 50% of the visible network for several years now - and ironically the datacenter perf issues (unrelated to Synapse) we had over the last few months have shifted that balance further - it’s about 35% and dropping. Ideally we will turn it off entirely once we have decentralised accounts.
> lack of e2ee by default is iffy
Our main project right now is to fix this. Cross-signing is mid flight; E2E search is done; Pantalaimon (E2E compat for dumb clients) is done; remaining key distribution bugs are in flight. We’re aiming to turn it in by default in Jan.
> “I couldn’t figure out how the state resolution works”
State resolution is the main technical novelty in Matrix, and yeah - it’s hard, much like git’s merge resolution is not exactly easy either. Unfortunately it comes with the territory; if you want to have consistent room state while replicating them over a byzantine network of servers to stop hijacks and other abuse, you have a relatively hard problem to solve. We got it wrong the first time; the current version gets it right (as far as we know).
The spec (which is deliberately formal and terse) is https://matrix.org/docs/spec/rooms/v2
However, there are supporting documents linked from the spec to help clarify: the original spec proposal at https://github.com/matrix-org/matrix-doc/blob/erikj/state_re... and the guide at https://matrix.uhoreg.ca/stateres/reloaded.html etc
Ironically I think that state res is one of the best documented and understood bits of Matrix now (which is just as well, given how important it is).
After more than two years in and around this space I'm still comfortable with stating that I'd take Matrix with all^wmost its flaws over ActivityPub. All of these protocols kind of suck. I see Matrix as having the lowest hamming distance between where it is and where it should be.
