Leveling techniqe for wooden floors
This is not woodworking per se, but.... I would recommend mixing mortar and creating a level surface at one level in which the subfloor can be firmly supported and either anchored or glued.  If the base concrete is loose, it should be removed down to a firm base and then refilled to the appropriate level.  Note that anchoring to the concrete may be difficult with nails and may require concrete screws in drilled openings to provide a secure base.

The pitfalls are instability from not providing a firm anchorage to support the flooring and the possibility of mold or decay if the wood comes in contact with water or excessive humidity below.