Yes, there are many omissions on the US censuses, but don't jump to conclusions and think that your ancestors were missed if you can't find them. Most of the time I've finally found missing ancestors after searching different spellings of their last names, by searching other relatives, or by trying to follow their neighbors from 10 years before. I have a few more tricks I use. At times I have even paged through every township in a county until I finally found my family with an unreadable surname.
It surprises me how thorough the census takers were even if they misspelled names and botched information. So far I’ve even found people in the deepest, poorest hollows of Appalachia. Usually the individuals are in the census somewhere, so I wouldn't give up too easily.