Another issue is that you still need the requisite number of nested elements to hang things off, e.g. for panels - you need a panel heading element with a heading inside, then a panel body div etc., and all of this is likely contained inside layers of row and column divs, regardless of whether you've used less or sass to reassign the styles -- you're still stuck with a hierarchy of elements that's prescriptive.
Another issue is that you still need the requisite number of nested elements to hang things off, e.g. for panels - you need a panel heading element with a heading inside, then a panel body div etc., and all of this is likely contained inside layers of row and column divs, regardless of whether you've used less or sass to reassign the styles -- you're still stuck with a hierarchy of elements that's prescriptive.