Of course it depends on what head to head comparison you are making but:
Shuns have three drawbacks that I see:
1) Shun uses VG-10, by and large, which is a good but not great cutlery steel, this is the same steel used in the Tojiro DP at a considerable savings.
2) Shuns are overpriced for the performance they deliver. They are not bad knives, but you can get good knives for cheaper, or amazing knives for the same money.
3) The ubiquity of Shun at American kitchen retailers makes them . . . less unique and exciting than some of these less well known manufacturers.
Macs have two:
1) I am not a steel guru, but I understand the Mac steel is, again, good but not great. That being said, I own a Mac Superior santoku, I love it, it does a great job. Part of the Mac cult, I think, is they are thin and that contributes a lot to perceived sharpness.
2) Macs have a utilitarian aesthetic. This is not a huge drawback, the Richmond Artifex line has a similar aesthetic, but for the same price Fujiwaras and Tojiros offer western style bolsters which up their game a bit.
This is all preference though and the Macs and Shun are not crap. I think many on this forum tend to prefer something different for the same money, but I would guess that both Shun and Mac are still best sellers on CKTG.
BTW Mark has been trumpeting the value of this Shun: http://www.chefknivestogo.com/shunsora8chef.html
The handle is a bit chintzy, but the knife is attractive and a good performer at a competitive price.