I personally recommend engineered wood floors over slabs and leave solid wood floors for raised foundations with wood subfloors.
Here's the thought. Concrete absorbs moisture during the wet season and dries out during the dry seasons. While this is normally a minor thing, some locations can experience severe climatic changes from season to season, resulting is radical moisture differentials. The problem with permanently affixing the flooring to a slab is expansion and contraction issues. This is why a "floating" floor is IMHO a better choice. Secondary benefits are that a floating floor is faster, easier, and cheaper to install than a permanently affixed floor, especially over concrete. There is also the issue of clearances under doors throughout the house when factoring in adding a plywood layer over the slab, plus the flooring itself. None of these issues generally exist with a floating floor.
This is not to say that a raised foundation/wood subfloor doesn't experience moisture swings, however, generally, most raised foundations don't have sub area moisture issues. If you have a raised foundation and you have moisture issues, then you need to address the moisture before you address your flooring.
With a wood subfloor, it is perfectly acceptable and reasonable to mechanically install the flooring - nailing to subfloor. The only caveat is that the flooring be rated for mechanical fastening, which most solid wood flooring is. Engineered flooring usually needs to float.
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