I never really thought that it was the marines who had the hard time of it. For me, it was always the stealers who have the challenge.
Stealers are a one trick pony. Looking at 3rd Ed, terminators have a variety of tactics available - they can shoot, overwatch, guard, use different ranged weapons, different CC weapons, psychic powers etc. With good positioning, you can create fire lanes, choke points, cross fires, defensive perimiters... Genestealers, on the otherhand, are just good at CC. They really are at a strong disadvantage, even with the numbers. After playing the missions several times, the marines have it pretty easy, as they know where to go, and when.
Made the stealer's job a lot more tactical, as they can't rely on fancy gadgets to win through!