This photo shows two common flaws of decks. One, the support posts are too tall for their size. These will be prone to flexing. Best practice now stipulates a minimum of 6 x 6 posts, with additional framing possibly needed in tall construction such as this.
Second, there does not appear to be any diagonal bracing to stabilize the deck if side loaded conditions. For instance, if a considerable number of people are on the deck they could exert enough momentum to cause the deck to shift to the left or right. Without adequate diagonal bracing, that condition at this height would likely result in some serious injuries.