Aside from the obvious fact that Wheeler had the misfortune of hijacking Veitch's potentially legendary run on Swamp Thing, and the pitfalls of the art*, what exactly is so bad about this run? I have read summaries of the various issues and it seems to continue the same traditions as Moore and Veitch's Swamp Thing and it introduced the current iteration of Tefe Holland.
*the two major complaints I have seen, which say nothing of Wheeler's actual writing.