The short answer is yes, this is a fairly secure connection. It can be made more secure by using any number of additional features within either the sshd service or nxserver service. I’m including a link to a webpage that covers some of these options in better detail than I can. It is not the only good source for this type of information available, but it does give you a lot of information that you can use to decide what, if any, additional methods you would want to use to further secure a connection coming in from the Internet. Good luck with it.