The one thing that I will say is that anyone who gets the link to an Unlisted synth or pano does not need to have a password to see it, so for anyone who has the link, it is basically public. If the people you send it to decide to send that link to others, then people you didn't intend to see it might be able to see it.
Similarly, if your synth or pano has been public for a while, then people may have already saved the link to it (like Google and Bing) + it may take some time for that link to go away, so unless you have comments on that version of the synth or pano that you want to keep, you may want to just delete the current version + reupload it as an unlisted pano or synth. That way, only people you choose to send the link to the new version (with its new URL) will be able to see it.
If there are comments on the synth or pano or it has been favorited or has a high view count, then you can at least set it as 'unlisted' to keep new people from finding it.