It's just, not fun.
Firefights should be fun and exciting, but they're railgun fights instead. RO has deadly weapons too, but you have a high chance to survive that fight if you know what you're doing. I think this is because of the huge hitboxes in INS. Our beloved RO has hit detection per pixel, so you can have a bullet go through your legs, between your arm and body etc. This makes the fights very realistic.
BTW I don't think COD4 is "way realistic" than INS, even in hardcore mode, it's still a fast paced deathmatch. INS has a slower gameplay, realistic maps, but somehow boring firefights.