Is fan fold insulation an acceptable underlayment for hardwood?
-Is fan fold insulation an acceptable underlayment for hardwood?


As a complete underlayment for snap-lock flooring, yes but I'd be more comfortable using rolls similar to what the manufacturers specify.
Hardwood flooring installed traditionally with nails is to be installed directly on the subfloor. (Rosin paper is optional)


-But it is being used as a leveler from one floor level to another.


Then no, don't count on foam to make a good shim. Use appropriately thick plywood to shim any low areas. If 1/8" sheets would be to thick, use a planer to bevel the offending area of the floor, bringing it down to an acceptable fudge (the foam is for fudging any discrepancies; not a blatant mismatch of floor levels).