They are only as strong as what you have them bolted to. That is why I replaced my rotted out rocker panels before installing the sliders. They bolt to the uniframe, leaf spring box, pinch seam of the rocker and the fender. If all of these are solid I am hoping not to have issues. If any of these are not solid then you will have a weak point.
If you search Google for JCR Ultimate Slider Review you will find some threads about it.