It's not because the game is designed brilliantly and the enemy AI is good, because those things are far from being true.
A couple of hits from an enemy machine gun will take you down instantly, and the enemy soldiers are a bit too accurate with their shots. Even on the lowest difficulty settings , the enemy soldiers could shoot the wings off a fly from 2 miles away... with a pistol. Even the friendly AI is basically non-existent. Friendly units are terrible at finding cover and will sometimes just stand in the open and get shot repeatedly , waiting for your orders.
What makes the game even more difficult and/or frustrating is the lack of a cover system. Most of the time, you will never be quite sure if the enemy soldiers can see you or not , while you're ducking behind a small fence.
Even the levels designs in the game are not impressive at all. The levels in the game just don't anything interesting about them, the levels just seem uninspired.
It seems odd that a game that has such brutal enemies and emphasizes cover so much would not have some kind of a cover system.
The story in the game is not particularly good either. The short version of it is - mexican rebels are angry , you have to calm them down. That's pretty much it. It does a really poor job of making you care about any of the characters or things that you're doing. The game tries to make you care about certain characters several times during the game, but these attempts fall flat every single time.
Though it's not all bad. The game has a semi-decent multiplayer component and the game looks rather nice.
If you're a fan of brutally frustrating games and are used to pressing the "quick save" button - go ahead and give this game a go. if you're not - don't .