It bugs the hell out of me that they apply styling to vanilla tables. I wish they'd namespace it with a class because it messes up things like datepickers on JQuery-UI.
* styling of default elements (like table) interferes with other styles
* inconsistent use of variables
* encouraging users to define columns with html classes (span4, offset6) instead of using less to apply the rule to the html.
* completely incompatible with JQuery UI (they had an open bug about it with some workarounds)
In the end, I ended up just picking a few good ideas out of it. It looks great and it's probably ok for a certain class of sites but be aware of the limitations as well.
That doesn't, however, mean they don't inherit from the style of the base element. Which they will, no matter how they're classed or ID'd, if the base elements have been styled in CSS.