I would use them if the cost is not too high. ANY post in the ground will eventually rot. It just depends on how long it takes to do it. The better posts may last 50 years but if they are in the ground they will eventually rot. Cheaper post may not last 15 years.
The cost to repair down the road is HIGH. Also if your sire gets much wind I have seen sheds/barn blown over after the post rot just below the ground level.
Look at RR ties. They are the heaviest treated thing around and they rot laying in gravel. I know they are usually some junk timber but it still proves the point.