Everything is unencrypted so anyone between you and the server can change or drop packets. Theoretically mods shouldn't be able to attack the client but they have quite a lot of control over the server and other mods.
The biggest threat to me is the unencrypted stream. The other issue is that mods may have security issues that allow a player to crash the server or do something nasty. Last time I checked there was some work being done to secure Minetest better but I don't know what the status is.
Sorry for my somewhat unrelated comment (I unfortunately do not know of any explicitly safe servers :( ), but if anyone's interested in making a server, consider using the following mod which adds pronoun tags to players. I was disappointed to see that no server that I tried was using it (or perhaps it just wasn't advertised or presented in a GUI interface). (I haven't tested it myself, but it seems like a feature every server should have)