I believe the problem stems from the fact that Mathews purports to be independent and unbiased. This bugs people for the simple fact that it is a lie.
Now take Hannity, he consistently repeats that he is a Reagan Republican and makes no excuses or lies for being biased in that direction..in other words, he's up front about it.
You may be right in your criticism of Matthews on this basis. Frankly, I don't watch him that much. I do know that I have had questions as to which side of the fence he is on. I swear, sometimes he seems to be espousing conservative viewpoints.
Hannity is, well - Hannity. You are right there as well. He makes no excuses for his biases, which I respect. What I don't respect about Hannity is the half truths, lies and distortions he utilizes on a regular basis.