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I am entirely sure that this is definitely NOT the case. The reason I sound so confident is that I have written a test for that which has run on the 48 combinations of {Windows 8, Max OS X Mavericks, Ubuntu Server 14.04} x {PHP 5.3, 5.4, 5.5, 5.6} x {Joomla! 2.5, 3.1, 3.2, 3.3}. In fact it has not just run once, it has run ten times on each environment over the last week. The Secret URL Parameter works perfectly fine and I'm willing to place a bet on its working fine. That's why I barely slept in 3 weeks, writing automated tests for Admin Tools and testing the tests on all supported environments: so that I can bet that a feature is REALLY working, not just me thinking it is working.

For your information:

• You cannot access the administrator URL without a secret URL parameter UNLESS you had previously accessed the administrator URL WITH a secret URL parameter from the same browser. In the latter case the "this guy knows the secret URL parameter, don't kick him out" flag is already set in your browsing session. A bug in 3.1.1 and earlier (the same one I write about in the next bullet point) prevented this from happening.
• Version 3.1.1 and earlier had a bug when logging out. You were kicked to the front-end and a security exception was raised. This was wrong. When you log out you should see the login page without a security exception being raised. This is what you observe in 3.3.0 which means that I did fix the bug.

Try clearing your browser cookies and then retry accessing your administrator URL without a URL parameter. It will kick you out to the front-end of the site. If it doesn't kick you out the update of Admin Tools failed. In this case just reinstall Admin Tools 3.3.0 without uninstalling it first.

BTW: I seriously doubt you are running Joomla! 3.6.6. Maybe you mean 3.3.6, the latest published Joomla! 3 version? In this case you are using the same Joomla! version I tested against.

We have already tested Admin Tools with nearly 300 tests on each of the 48 supported combinations of Operating System, PHP versions and Joomla! versions. We cannot reproduce this issue in any of these environments. Therefore we consider it to not be a bug but something related either to a partial upgrade of your Admin Tools installation or something related to your host. Unfortunately neither of these factors is in our control and we cannot provide any further assistance.