A Realtor from a Xenia office said to me recently that there had only been one deck in the last two years that I had not thrown the book at. I got to thinking about that and realized he was probably correct. Unfortunately most of the decks we inspect, even professionally built ones are badly built and potentially dangerous.
One of the biggest issues are main beams being undersized for the potential load and the fact that they are nailed or screwed rather than bolted to the posts.
Stairs supported by dropped headers are a big problem.
While not required by code in our area, the posts should be notched to take the weight.