I have been taking this stuff in one form or another for 4 years, and I can tell you from firsthand experience that it does NOT kill your good flora. That is also backed up by what Jim Humble says.
In fact, it has exactly the OPPOSITE effect. It promotes your good flora very much; how could it do this?
Simple: it kills off the bad stuff by the bucketload ... what happens then? Well, nature abhors a void, so in move the good flora as soon as the opportunity arises!