the government is not liable.

It's like saying "we encourage you bring your own lawyer. however, if you choose to use our lawyer, you are welcome to do so at your own risk."

As to the evidence, I think one of the jobs of the defense attorney is to challenge the validity of the evidence, including how it is collected and how it is interpreted.

So if the accused loses the case, it's his/his teams fault that they can't convince the jury that he is innocent. The governement is just the opponent in the game, the jury is the judge.

I think this is why you can't ask the government for compensation unless it didnot behave in good faith as Hunhun said.