is factory reset enough?

After buying a used phone, is just factory resetting it enough? I remember reading an article where it says for Android phones, personal information can still be obtained even after factory resetting it. I want the used phone to be as if it came fresh out of box, non-hardwarewise. Would factory resetting suffice, or are there better ways to wipe everything clean? Thank you.

 

how would I wipe the phone? What's the difference between hard reset and just doing factory reset from the settings?