I believe a big part of the perils of being sold down the river had to do with disease, particularly malaria. Louisiana is basically a big swamp, and mortality rates from malaria were very high for both slaves and others. The slaves of course had no recourse from the mosquitoes.

The area around New Orleans had a lot of sugar cane plantations as well as cotton plantations. The sugar cane plantations had a very bad reputation as a death sentence.

Here is a section from a book discussing this

In short, Yes. This is of course not to minimze the difficulty of slavery in the american south. However the conditions in sugar cane plants in the carribian and south America were quite terrible and it was not uncommon to be literally worked to death. Brazil was an infamous example of such conditions. To give you a comparisson there existed a separate set of sugar bought by people in America, similar to how we would think of "fair trade" coffee, the conditions of slavery for those working cane farms were largely recognized as particularly horrendous.

In the book 12 Years a Slave, the author worked on both cotton and cane plantations. They both sounded awful in their own way but he does not say one was worse than another.

However, demand for slaves was very high there as people were constantly claiming new land to farm, land that was rich and had lots of topsoil. Land was cheap--$1.50 to $5 per acre, while a healthy male slave who could work that land went for perhaps $1700 in the mid-1800s. It was backbreaking work and because the weather was warm, they could harvest multiple crops a year. So the work was very exhausting and difficult. They had malaria there at the time, too. These factors made being a slave there even tougher. Plus, being sold down the river meant never seeing your family and friends again. With top dollar being paid there, if you were going to sell a slave, for all these reasons, being sold down the river was very desirable for salve owners and very undesirable for slaves.