This Will Result In The Floor Being Hollow
Hardwood floors that are not installed properly tend to have large cracks or spaces beneath the hardwood paneling. This will result in the floor being hollow, and will be very noisy when someone walks over the floor. It can also end up being damaged, as the air pockets beneath the flooring are not sufficient to support the weight of a human walking on the floor. You could end up cracking your hardwood floor if the floor is not installed properly.
When a professional installs the hardwood, he ensures that there are no bumps. Non-professionals may not have sufficient experience to notice and remove any bumps, especially when installing hardwood on the stairs or steps of the house.