As the other said I was looking for repositioning cruises like 2 months in advance, from one continent to the other. Like I did Europe -> South America 550 euros Costa Cruise last year and Miami -> England NCL 650 euros early this year
The difficulty is finding good prices for a solo cabin. For a cabin of 2 it's even easier to find good prices for repositioning cruises
As the other said, there are more days at sea (less stopovers), and it can happen that some stopover port get cancelled (for example they cancelled my stop in the Azores because they didn't want to bother entering a big Atlantic storm, it was risky)
The difficulty is finding good prices for a solo cabin. For a cabin of 2 it's even easier to find good prices for repositioning cruises
As the other said, there are more days at sea (less stopovers), and it can happen that some stopover port get cancelled (for example they cancelled my stop in the Azores because they didn't want to bother entering a big Atlantic storm, it was risky)