While some people do, I am not choosing what sources are biased based on whether their findings align with my beliefs. I am choosing based on whether their results consistently align with a certain ideology or political stance, what their goals are, and where their money comes from.

The Manhattan Institute consistently publishes findings that align with American conservatism, their goal is explicitly to influence policy, and while we don't know exactly where their money comes from because they don't make that information public (which I think is generally a bad sign), information revealed by tax filings shows that most substantial donations are from conservatives. This is why I consider them to be biased.

I don't have any specific criticism of the study, because I did not spend the time to engage with it. I have limited lifetime, and I must choose where to spend it based on what is likely to be fruitful. I do not consider engaging with articles/studies from biased think tanks, whether liberal or conservative, likely to be fruitful.