One friend told me that economics as a field has a huge "conservative bias", and you can't trust their methods because of this reason.

Which is strange, since (according to The Making of an Economist, Redux by David Colander) only 16% of economists are conservative (while 47% are liberal, 24% are moderate, and 6% are radical). Perhaps he meant that the field of economics has less of a non-conservative bias than other fields (like sociology and political science), which I'm pretty sure is true.