"Ah... finally!" I thought, thinking about a customer Joomla! 1.5 database which really needed to been cleaned up. Resolving this problem manually over years, it wasn´t such a great pain, but it had to be done and thought off.
So I installed a testsite, just to get a feeling for the plugin. Fair enough, its easy to handle... to easy for my taste. Let´s have a look at the most critical setting, the "Exclude User Names". Ok, I do know my "special" users, but how many don´t?
In my case I have a user "Msg-Bot", which sends Newsletters. This user would have been deleted, as he NEVER completed activation and NEVER logged in, he is ID 1, so he doesn´t have to. And whats about the CBE-Userlist? Ok, the CBE running on this system isn´t the actual version for Joomla! 1.5, but I could imagine, that this didn´t change. Just deleting jos_users and jos_core_acl_aro would get CBE out of sync and would produce a long list of dead profiles (in my case about 2.500). Thinking about an Joomsocial site brings up the same question.
I find it astonishing, that there is no rollback funktion, there is no manuall control possibility before users get deleted. I really do not recommend this plugin as a "fire and forget" extension, mostly if you have something more than just a Joomla! vanilla setup. In my case, I did it manually and tried to contact you at your forum but did not received an activation link until now.