why did they told you not to use signal

Matrix.org is centralized like Signal (you can say Matrix is not centralized on paper, but in practice this isn’t remotely true). Both are stockpiling metadata in the West… what’s worse is Matrix’s eventual consistency model means syncing metadata to all servers is a by-design requirement (& also why all servers & clients are slow). There are options like Snikket to take all the hard parts of self-hosting out of the equation, but finding someone you can trust to host a server might be worthwhile. I would be wary of anything centralized.

Matrix/Element is pretty private, but not wide spreaded. For the use with friends and Family is more realisticto use Signal or any other decentralized Chat.

Private against who?

Privacy communities need to really drill in the idea of threat models instead of pretending privacy is some linear scale and the ultimate goal is to bury your phone and computer in a lead-lined concrete block underground. Privacy and security are meaningless concepts unless you know who your are protecting it from and what their capabilities might be. I don't need to hide from NSA Tailored Access Operations because I'm not trying to x the y of the USA. I do need to protect myself from basic scam attackers, copyright trolls and neo-nazi stalkers. And Matrix, along with certain basic opsec guidelines, does that and more for me.

both are good, even Signal. For private conversations, you only need to avoid Telegram and other obvious ones

Matrix and Simplex is fine but I would recommend Signal for family and friends. Threema is also option but not user friendly for friends and family who wants easy user discovery than sharing userIDs.

Matrix isn't more secure/private than Signal. Both have advantages and disadvantages. Signal has a centralized server, but has no access to the keys to decrypt any of the data flowing through them. Matrix chat rooms live on servers that would theoretically be able to access the data in the rooms, so you need to trust the server owners. Advantage is that multiple servers are involved so no one sever can kill your chat room. With Signal, the disadvantage is if you join a chat room, you can't see any past messages because those are encrypted with keys you don't have access to. Similarly if you move to a new device, that device won't have any of your past conversations because the new device doesn't have the keys for those messages. (though migration is now somewhat possible but done poorly IMHO).

So, they address different concerns. Is your concern keeping your conversations private, or keeping your conversations from being censored? Signal is more secure and private, but more centralized and easier or to fail. Matrix can be secure if you host your own server or explicitly trust the owners of all servers that house your chatrooms to keep them secure and to not sell their servers in the future. Matrix is more distributed, so more difficult to be censored or have your data lost by a single point of failure.

Is it "secure enough" depends on what your concerns are. If you host your own, then it's as secure as you are technically able to keep them secure yourself. Otherwise it depends on the server owner.

If it's low privacy needs (ie you don't have a state threat model), Signal is completely fine. I use it to talk to my friends. I also use Matrix, though federated Matrix isn't the best for privacy either due to the amount of metadata that leaks through federation. But federated Matrix is also fine for the kinds of things you would use eg Discord or IRC for.

If you do have a state threat model, I personally think SimpleX is ideal for that, but it doesn't have as much of a userbase so you probably need people who care enough (eg people actively under threat) to switch to a new platform. Whereas most people I know are already on either Signal or Matrix, and I'm not having particularly sensitive conversations with them either so both work fine.

In signal, You can turn off phone number visibility and make it so that you are only searchable by username or qr code. Yes, it's centralized, but signal is a nonprofit project with generally good guiding ideals. I use matrix for some things and signal for everything else.

Matrix is great, you can use another instance though.

https://servers.joinmatrix.org/

you don't need to use matrix.org. there are several open homeservers, like chat.mozilla.org, but also there are people who host services for others to use. you may have a look at current lemmy hosts, and their other services if they have them.

I am really concerned about the dominance of the central instance on Matrix. It has visibility into pretty much every groupchat - if not in content because of encryption, then in all the metadata. I'd rather use another public homeserver.