I don't want to spoil anything so I'll be vague. The problem is that the main character went from being a conniving underdog to the top of the stack way too quickly. Now the stakes are much lower because he already got what he wanted, and it's much less interesting to watch.
If you look at the British series, it's much shorter, and for good reason.
I feel like they butchered the character by making him 'angsty', and I personally hated the way the 3rd season ended. It exemplified everything I disliked about the 3rd season.
The opening scene for the season 1 had me hooked. They defined that character so well with that opening scene and I was immediately interested to see what this character was willing to do.
Then season 3 happened and I find he's turned into every other character in every other modern show.
I wish they had taken the approach of just making him ruthless towards the other nations in achieving the US's goals instead of the direction they went in.
If you look at the British series, it's much shorter, and for good reason.