How to evaluate the quality of a conference that has a high ERA ranking, but whose organizer has a lot of negative reviews online? In Australia we follow something known as the ERA ranking, when submitting papers to journals and conferences. My lab stresses publication in venues which are graded as only A or A(star), as per this system ( ERA-A increases possibilities of funding). In the area of biomedical engineering there is a specific conference which I found is ranked as ERA-A. Not a lot of international biomedical conferences are ERA A-ranked. 

The problem is I found a lot of negative reviews about the conference organizers online and therefore am skeptical about sending my paper there. Yet, I also know that an ERA-A rank publication very early in my doctoral program can make a good impact on the committee, when my doctoral assessment review comes up later this year. Moreover, this will release some pressure/tension in terms of the doctoral review and I could go on to work on larger targets without worrying about the doctoral assessment ( since it would be considered good progress if I can show an ERA-A paper in the first year of my program). 

However much I try, I have failed to understand how this conference gets a top australian rank, when there are so many negative things said online about these conference organizers. One possibility, is that this particular conference is the only one which may be famous. The other is a lot australian academics might have been ranking it as ERA-A. But if so, why only Australia? What does the rest of the world say? The third possibility is that the online reviews are dubious claims made to malign the organisation.

How to evaluate the quality of a conference like this?
You shouldn't take the ERA rankings too seriously.  Ranking ten of thousands of journals was an enormous job, and they did amazingly well given the size of the task.  However, in the process they made a few questionable judgment calls and every once in a while an outright mistake.  

For example, in my field of mathematics, the list of A-rated journals looks rather good overall, but there are a few surprises and at least one journal I am convinced doesn't remotely deserve an A (Fuzzy Sets and Systems).

I'd recommend asking your advisor or other faculty members for their opinion about the IASTED biomedical engineering conference (rather than relying on opinions from random people on the internet).  If they are familiar with it, then they should have an opinion about whether the ERA A rating is well deserved.  If they aren't familiar with it, then that is itself a bad sign.  Not being familiar with a C journal in your specialty is understandable, but not being familiar with a genuine A journal would be more surprising.

You can also investigate it yourself.  Have they published any papers you've read or seen cited?  Can you find any papers in their proceedings that impress or excite you?  If so, then at least your paper would be in good company.  If not, then that's another bad sign.