Is it safe

Yes. Openport is the safest reverse ssh solution on the internet. Why? Several security measures have been put in place:

Port scanning is detected and blocked
users must click a link first, before the port is opened for their IP (You can change this in your account settings)
users cannot make more than 10 connections per minute to your port (You can change this in your account settings)
Http-forward links are interpreted and reconstructed by the openport servers first, which means you are not vulnerable, even with old/unpatched webservers on your end