What, exactly, constitutes reasonable suspicion? How can anyone have any "reasonable suspicion" about a person's immigration status and not include some enormous number of citizens? The day that an American citizen with a "Hispanic complexion" and a "Latino accent" gets detained for not producing proof of citizenship will be the beginning of the end for this law, so what's the point of it anyways?
First of all, even if we granted that there is NO possible fair application of 8B (reasonable suspicion) the law would still be in good shape. The example you offer might be the end of an officer's career, but since such a detention would not be in compliance with this law it obviously wouldn't spell any sort of doom for the law... at least not in the courts.
Meanwhile, the other provisions of the law still function perfectly well. Illegals will be deported if they are convicted of a crime (or, more accurately, turned over to ICE). More importantly, this gives AZ law enforcement the right to arrest, without warrant (but with probable cause), any illegals previously convicted of crimes in AZ (8E). So if a drug dealer gets convicted, deported, and then crawls back into AZ, he can be arrested immediately and sent back to ICE.
But reasonable suspicion, as has been stated, is clear and articulable facts not pertaining to race, ethnicity or language. How might this be applied? I don't know, but I bet we'll find out. It could be a van full of people without IDs near the border, someone who cannot articulate his immigration status (if it came up legally in an interview), or anyone who cannot produce a valid form of ID.