One thing is for sure, do your own research. But something that isn't said in that little article, but can be gleamed from it. Those "studies" are isolating a single compound from the entire plant. And when they used that isolated compound in large quantities it caused negative effects. I'm sure that if someone wanted to look for specific things to isolate in any plant they could find something toxic when taken in massive quantities. Even an organic carrot has compounds that can be isolated and used as rat poison. There's a reason the pharmaceutical sponsored research centers decided to look at this plant in particular. It works, and it interferes with what they're doing.
I could also get on the net and dig up articles on people who used comfrey safely while working with their doctors who were astounded at the rapid rate that they recovered from torn ligaments. So there's definitely two sides of the coin. But one side of the coin has nothing to gain. So again, do your own research....it's worth it.