Many organs have overlapping HU intensities. Some are quite distinct (e.g. lungs, bones), but most others will have a good deal of overlap. Also, there is noise and anatomical structure (e.g. each organ has its own set of blood vessels) which interfere with intensity based segmentation. Also, whenever segmentation is based on intensities, it is hard to get away without some level of morphological operations (opening, closing) or connected component analysis (removal of small islands of noise).
You could take advantage of prior knowledge that the image will contain an entire human. If you approximately segment the lungs, bones and surrounding air (easy), you could deduce approximate locations of other organs from that (brain, heart, bladder etc). That should help a lot with disambiguation of overlapping HU intensities.
And if your goal is not exact segmentation, this might be enough.