Yes, it is harmful. While clearing out your stuff is great. I am a fan of it myself, formatting or resetting a device often is not good for it.
You would be better off just clearing out/deleting the things you want to remove. When you remove them and then add more, the space is overwritten. Most people could never access the old stuff.
And the risk, there are a few. The main one being that the wear on the storage itself will, over time, make there start to be errors.