We all know we could reduce crime by putting cameras in people's homes. But we don't do that, because there are risks to those kinds of invasive measures.
Aadhar is infrastructure that allows for centralized control of the population. Sometimes that control will be used for good things, like stamping out bribery. But that control can just as easily be used for evil too. And history has shown time and time again that's exactly what happens. I live in Canada, a country filled with refugees of authoritarian regimes. Among my friends I personally know lots of people who have fled those kinds of regimes; I know very few people who have fled countries because of bribery. The latter is annoying and unproductive, but the former is deadly.
That's why you're seeing so much opposition to Aadhar: because it's incredibly dangerous.
edit: better wording