This guy has major swag. No, seriously, his reputation is off the charts, and that's why Machiavelli is talking about him even though he's responsible for half of Italy's problems. Read this:
One might almost describe him as a ruler new to power because from being a weak king he has become the most famous and honoured of Christendom, and when you look at his achievements you find they are all remarkable and some of them extraordinary. (21.1)
Seriously? He's so awesome that it's almost like he made up a whole new kingdom with his greatness?
When Ferdinand comes up, Machiavelli is talking about how important it is for a ruler to have a good reputation, and he certainly knew how to do that. It was almost like he never slept.
Ferdinand was always taking over some place or doing some new thing that kept both the people of Spain and his enemies on their toes. He never took too long to do anything, and they never knew what he was going to do next. So his people were "in a state of suspense and admiration, concentrated as they were on the outcome of his various campaigns" and "he never gave the more powerful men in the country any slack time between wars when they could plot against him" (21.1). With that kind of work ethic, Ferdinand made Spain the most powerful country in Europe.
To top it off, his grandson Charles was actually a decent ruler and since he was Holy Roman Emperor and all, he was even more powerful.