Trust is the most important variable here. Do you trust the
person who runs the echoServer that you're using? If you
do, then your connection is secure.

For added security, you can add 128-bit AES end-to-end encryption
to the EchoVNC data streams. There's a smal tradeoff between "data
security" and "performance", but it should be optimized.

Still, all the encryption in the world won't help if you just don't
trust the person who runs the echoServer. If this is an issue for you,
solve that part first.