The really big issue if you use only fendenazole or Panacur is that you may not be killing all the strongyles. I was getting a lot of strongyles showing up on fecals with Panacur. I now usually rotate between strongid P and ivermectin. I have never had a bad reaction, but I did get a positive fecal floatation recently. I plan to retest about two weeks after using ivermectin again. I'll have to see how that goes.
I have also heard people say that their horses got sick after using Quest.