This topic has been debated on this forum for at least three years. Some, like OhMike, take the position that concrete attracts or traps moisture, and shortens the life of the post. Others have said that there is absolutely no evidence for this belief.
I have put in posts using three different methods, for what it is worth here is how the techniques performed.
I put in six 4x4 cedar posts in concrete 20 years ago for a fence. To date, the posts seem to be in perfect condition. I put in a 4x4 pressure treated mailbox post in gravel/soil mix 20 years ago, to date no issues.
The previous owner of my house installed two long cedar post fences in 1959 using direct burial in soil. After 50 years, all of the posts are rotten.
I built a deck using 6x6 PT posts installed on concrete sonotubes using Simpson standoff brackets. Only been two years, but so far no damage. I have seen similar deck designs where the posts have lasted at least 20 years.
I like the idea of using dry concrete mix, let the moisture in the soil wet the concrete. It might take a year to fully cure, but so what, you don't really need any strength. From my perspective, it seems that the concrete would harden right up against the post, leaving little or no room for moisture to penetrate against the wood. Even if a little moisture wicks through, I don't really see this as much of an issue. I would like to see a definitive study on whether embedding posts in concrete is really a problem, I have not found one yet.