The LII is literally just an accessible form of the various federal codes, statutes, regulations, and certain administrative rulings. It can be verified against paper copies or against paid services like Westlaw and Nexis.
The LII has been around for more than a decade. The LII is used by progressives, conservatives, libertarians, socialists, the current (Trump) administration and the former (Obama) administration, all with no complaints. It has been used by lawyers and judges on every side of the aisle. If there was any bias, it would have come out by now.