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.