My experience with this type of structure support is two-fold:
The first is drainage- regardless of how much precaution you take, there's always going to be moisture penetration into the hole that can lead to rot- regardless of the rating of the material.
The second is replacement- if rot or damage does occur, it's a ***** to get that post out to replace it.
I much prefer as ICC requires (as per their deck codes), footings above finished grade and metal stand-offs that keep the post from direct contact along with a mechanical attachment.