Matrix.org/Riot.im has all the encryption you could wish for, a modern, useful interface, and a federated model in which everyone can run their own server and talk to everyone else, just like email.
Matrix doesn’t protect metadata on the server currently - so you have to trust the admins that run the servers you are participating in. In the longer term we want to fix this (https://matrix.org/~matthew/2016-12-22%20Matrix%20Balancing%...) but we haven’t started on it yet.
As far as I remember you needed quite big servers if you wanted to "federate" with others, like join big chatroom because Matrix will try to replicate the history and keep it in sync. Is it still the case?
Yes, if you want to participate in rooms with >10K users or >500 servers you need quite a large box (several GB of RAM) - although over the last few weeks we had several massive algorithmic performance breakthroughs which should help this a lot. these are currently being tested and implemented in Synapse (the python impl).
What metadata mitigations does matrix have (the point of Tor Messenger)? We already have a federated protocol called XMPP. Sometimes you are interested in not revealing who you are talking to.
That won't be enough for the average Tor Messenger user. Email's failings were the impetus behind both instant messaging and Tor. Users don't want/need federated models. Security aside, they want a convenient little app that will receive messages instantly while online but doesn't have to remain online 24/7.