Engineered hardwood over uneven basement subfloor?
I would be looking at getting some self leveling mortar/ concrete mix and at the very least, filling in those voids. Your sub floor needs to be securely fastened to the concrete floor which means it will "sink" in where the voids are and that will show when the new wood floor is installed. If you avoid fastening the sub floor into the voids hoping to just go over them, you'll feel the depression and hear a squishing sound every time you walk over it. The smoother and leveler your floor is, the better your new flooring will look and hold up. Good luck.