How to evaluate the quality of a conference that has a high ERA ranking, but whose organizer has a lot of negative reviews online?
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.