AOC and Bernie are some the most honest people in politics, it's what they are known for, fair and by the book, that's them to a tee
Eh... fairly or not, they're perceived as partisan and will therefore be considered biased.
But much more importantly, they have no legal experience. Which is kind of important for the person who is in theory the top lawyer in the federal government. The responsibilities are to prosecute and to give legal advice. I don't think they're qualified to do either.
It's not like being fair and tough is enough to handle a court case. The attorney general being someone who has never worked a case would be as far as I know unprecedented, and likely a train wreck.