You are correct in that there are no adverse health effects associated with MMW, and so long as they aren't saving images the privacy concerns with ATD are minimal, too...though the waste of time/money and ineffectiveness are legitimate reasons to oppose them.

I admit I tried it once at AMS, since these machines are the unique combination of MMW and ATD. However like many people I got a false positive and got molested anyway.

Now I opt out. First time I got an explanation of how the machine works, which I dismissed and got a pat-down afterwords. The next time we went through a "yes you have to...no I don't" exchange, but they relented and patted me down without the need to call a supervisor. Just be firm in opting out and you can get through.