I think you mean that the rear carrier legs are attached to the axle - not the skewers themselves.

If the legs are attached to the rear axle, then YES, it had better be safe. Your entire bike is loaded onto that axle. Whether the carrier legs are attached to the axle or to a tapped hole in the drop-out, the load goes onto the axle anyway.

I have had a number of bikes in which the rear carrier legs were mounted onto the rear axle and I rode them thousands of miles with no problem.

In fact, they had a better tendancy to stay on than the drop-out mounted carrier because the smaller bolts on the drop-outs would often come off and the carrier legs can fall into the spokes.