Consider me paranoid but I don't like services like this unless they come from reputable sources and even then I'd much rather run something local.
Remember to close your port in your firewall after running the test.
If an attacker wants to gather a list of vulnerable SSH servers in the wild they just have to map a bunch of random addresses (and that seems to be extremely common judging by the number of failed auths on my public server). Although it could be useful if you wanted to get a list of ssh servers not running on port 22.
But if it is private you're going to have to unlock a port, let the service do its thing and then re-lock afterwards. If you forget the last step you are now more at risk than before.
Also, since the service does not advertise what IP it will be connecting from beforehand (presumably the host you complete that form on, but that's not a certainty, it's IP is 40.112.150.31, in an MS Azure block) you would open up access to the world in order to do this.
The reputability of such service or even existence thereof does not have anything to do with how much your server is or is not secured.