I started hugelbeds two years ago and the hardwood is still pretty much in tact.
Hi,
a big concern I had while considering hugelkultur was that it needs water. You want the soil moist so it can decompose the logs. The wood helps with retaining water (it acts like a sponge), and building
berms helps retaining runoff water. However, a berm is not an ideal solution for a hot arid climate, since it heats and dries faster, especially if you aren't going to irrigate it, which is my case.
The solution I was pointed to, was to bury the logs
underground, and build a normal bed over them. Hopefully the soil will retain moisture better and logs will decay as expected.
I built it last year, and it was filled with rainwater in May, but I suspect it dried out completely by August. I can't comment yet on the decay rate of the logs, but I can affirm that the soil has retained moisture much better than normal beds. I've heard that it takes 10 years for big logs to turn into mulch.
Logs also require a source of Nitrogen to decay, so it may have been for the good that they hadn't decompose that much.
As for the soil health, I think that's a different issue. For a good soil health you need to add aerobic compost (fungal dominated is best for most crops) or compost tea, to protect it with mulch (living mulch is better) and to disturb it the less you can, all these while you provide the right amount of sun, water and wind protection.
The buried logs will help to achieve a good moisture level, but if you miss the other ingredients then your soil may improve slowlier.