I liked all three campaigns. It is a great game. I am a huge fan of realism in the games I play and OFP does this well.
But in contrast to the other posters here I found Red Hammer the easiest of all three. One of my favoruite mission is "Rage" (I think that is the title). I do love to move stealthiely, to kill silently and to plant loads of satchel charges and then watch in safe distance how the whole camp blows up in one huge explosion.
In contrast to others I think the story line in RH is not that bad or unrealistic or at least more so than the other two campaigns. Especially the development of Lukin. He is a loyal Soviet comrade. He does not want to shoot civilians which would be against official soviet doctrin, too. When he meets an old friend who was with the Soviet military and changed sides because Guba is not following orders from Moscow it is only natural that he changed sides, too. Moscow will not be pleased about what Guba did, since this could have sparked a world war. There was a reason why there were no direct military conflicts between the USSR and the USA. So any direct conflict could only be rogue forces engaging or like in resistance "supporting" a third party.
PS: Please excuse any writing mistakes I make. English is not my native language.