Non-slip carpet treads come with either an adhesive surface or carpet tape that sticks to the stairs surface. This is hands down, one of the most important factors that will determine how effective the treads will be.
If the carpet tape is of substandard quality, then the treads will start to peel off or shift, after a few days of use, which sort of negates the very purpose that you bought it for originally. So, you need something that sticks and stays put.
At the same time, the adhesive on the tape must peel off cleanly without leaving a gooey residue on the hardwood, because you will be adjusting it a lot when you first install it.