Is a descending aortic aneurysm more dangerous than an ascending aortic aneurysm?

Depends. Aneurysms are dangerous because many people don't know they have them until they rupture. Screening is important for people over 65 with family history of aneurysm, smoking or htn. Ascending aneurysms and para visceral (upper abdomen) aneurysms are probably the most challenging to repair.
Not, in general. Ascending aneurysm may affect the aortic valve or coronaries and treated in general under cardiopulmonary bypass. Aneurysm limited to descending thoracic does not have those risks. Endovascular repair can be done for daa but not for ascaa. Paralysis however is more of a problem with descaa. In any case, both are dangerous depending upon size, symptoms and related issues.