Be aware, hardwood flooring does not expand or contract equally in all directions. You will often see unevenness, but this is not at all a defect. It is just natural.
No sub-floors are absolutely level. It isn’t unusual to hear hollow sounds where the surface of the subflooring dips and ridges. This sound does not affect the integrity or installation of the hardwood and is also a natural thing that happens. Hollow sounds are normal and are one of the things that we get with the hardwood flooring. In no way is this a defect or lack of proper installation.
Exposure to sunlight will greatly increase the hardwoods natural tendency to fade, darken, or change shades over time. This is why window treatments are highly recommended as well as switching aroung your house furniture and area rugs in order to allow the floors to evenly age from UV exposure.