Singe-Player is hard and unfair. Multi-Player is marvelous.
So in the Single-Player you must conquer all of the countries and to do so, you can dispatch 6 pigs into the enemy base and kill all the enemy pigs. Your pigs can wield quite a wide range of weapons - depending on their rank. Surgeons can use healing items as well as items that cause bad status, such as paralyzing or poisoning items. If you chose the spy pig you can hide in the battlefield, you can use sniper rifles and you can also pickpocket your enemies - which is quite useful I might add. Bazookas and alike are also available if you chose to be a pyrotechnic pig and TNTs and mines are exclusive of the engineers. Throughout the campaign you earn points which you can use to promote your pigs. For instance, the surgeon branch has different classes and the as you get promoted (nurse, medic, surgeon, etc) you get new weapons as well as have more HP.
The campaign itself is quite hard. It starts easy: 3 enemy grunts against 3 of your grunts and there are lots of scattered weapons throughout the game you can use to your advance but then the enemy pigs are more advanced and have special perks, like, being protected by mines or being inside some of the special units the game has to offer, like turrets, which have high HP and a decent group of attacks. The medals you earn are based on how you fair in the missions. For instance, if you don't lose any pig, you earn one medal. Complete the mission, another medal. Collect a certain item in the game (usually protected or just hard to get) and you earn another medal. So unless you earn every single medal you possibly can, you can just start over, because you won't be able to get your pigs promoted. So, it's like, even if you promote the pigs by earning every medal, the game is still hard. If you didn't collect all the medals, it's just insanely hard.
Another fair or unfair aspect in the game is the enemy's luck, accuracy as well as yours. Certain weapons like the bazooka, deal weapon based on where you aim of course, and based on the power you chose for it. The more power, the further the bomb will explode, and there's no preview or redo, so at the first you will play using your luck and praying you chose not too much and not too few power, and pray the enemy doesn't (which doesn't, in the beginning) and as you gain practice, you will rely on your accuracy.
Enemies have a very lousy accuracy in the beginning, but around the middle their accuracy is about 90%. They almost never miss. So, you might find yourself out-skilled, if you didn't train enough.
In the Multi-Player, you can chose to play with (or against your friends, or the computer) in a wide range of scenarios. The Multi-Player is quite good and has a lot to offer, so you should take advantage of it, if not to practice aiming.
Hogs of war, despite being quite unfair, has a lot to offer and is definitely worth trying and is a wonderful experience to any "Worms" fan. I consider it to be great.