We do not have to imagine what harm trusting Palantir’s “Master Database” would make possible. Just look at the Horizon IT scandal. Horizon is/was an accounting software system used by the UK Post Office. The default position was that if Horizon said you had unbalanced accounts, then you were convicted, by Horizon.
Even when the Post Office discovered that Horizon was faulty and they were wrongfully convicting hundreds of sub-postmasters, ruining lives, causing deaths, the Post Office, which was invested in Horizon, they doubled down, continued to prosecute and destroy their innocent employees and engineered an horrendous cover up. And that was just the UK Post Office. They were wrong, but in the beginning at least, they were not trying to be malicious. The tRump administration is objectively malicious.
The last thing anyone with an ounce of sense wants is this kind of comprehensive database tool in the hands of the kakistocracy. It is insane. And it gets worse. Horizon was developed by Fujitsu. Fujitsu, a private company, left back-doors in Horizon, that they could access, presumably for maintenance, that allowed them to mess with and change/fabricate data. They even showed off this capability to people visiting their facilities. The idea that Palantir, named after a weapon of the Enemy, would not be able to fork around with your or anyone’s private data on such a system, is… naive.