On my gravel bike (steel frame, B17 saddle) I'm using an old Giant carbon post that I got with my first road bike back in 2005. Surprisingly it's still ok, but as I build up this bike over time I'm looking to add the ideal parts. Given this bike is mainly on gravel/dirt roads, does a carbon post really help with comfort, or is a Thompson or similar non carbon post same ride quality; I would say the latter would certainly have better durability.
There is no downside to a Thomson post. Your fat tires will take care of the "comfort." Some situations may need the set back version with the B17 because of the short rails.
The post I have on there now is a little offset, is the Thompson 0 degree? I don't have the height for the set back model. I have the velo orange setback post on another bike and like it, may get another one...
One thing I don't love about the Thomson setback post is that it has the same upper mechanism as the non-setback post. This means that the nose-down adjustment on the setback post, since it is tilted aft, is less than the standard post. I have a new Selle Anatomica saddle that has significant setback of it's own, and the Thomson setback post is almost at the limit of its forward tilt adjustment, according to the little scale, just getting the saddle level. There's a lot of extension showing in the rear bolt, of course, too. This may be a consideration for other users as well.