Basically a Space Marine is a literally super human badass that can shrug off almost anything your standard troop units have. Thing is you never fight just one Marine but at least a 10 man squad for minor problems to at least 100+ if they toss entire company's at you. Once you break pass a half dozen or so company's you are fighting a Crusade and have slightly bigger problem and your own counters if you're at all intelligent. So Marines can walk right through a hoard of Ork Boyz, Chaos Cultists, Guard Troops, and so on like a Main Battle Tank in a Napoleon firing line. Now most Elites worth their salt have weapons that can take one down but you're still trading Regiments of Stormtroops or whatnot for a Marine squad rather than Division for a luck hit by a Lemon Russ on one guy. They and other elites can beat on lesser troops all day long as they have the numbers, training, and weaponry to dish it without getting shafted if one or two of the enemy gets lucky.
Heroes can personally best whatever basic unit they fight till they run out of time to kill with their own two hands. If your hero is good enough then even Space Marine fall like wheat. So say Mr. McKillface slaughters 1000 men every round no matter if its peasants with pitchforks or Space Marines with plasma guns, you're going to run out of Marines long before you run out of pitchforks. However each hero is only one man. Be it the MC, a Chaos Champion, or a Guard Hero if they die is over, no matter if it's a lucky suicide bomb or a rival hero that bags them. So Heroes are the bane of Space Marines as they can go through them like the Space Marines cut through Guard troops. However if your willing to send the 100k chaff in your 10 billion man army to their deaths so one can luck out or weaken them for your own hero then you have plenty more were they came from. This is why Warbands are so dangerous. They have several to a few dozen or more Space Marines to kick in the regulars teeth, Champions to cut down the others Elites, and tons of chaff to tie down large sections of the enemy and wave attack heroes before the Champions swoop in if their not overwhelmed first.
Cheap units like Guard, Cultists, Boyz, and so on have only the most basic gear but one huge advantage. Numbers. It doesn't matter if you loss 40 to 1 KIA if you started with 300 to 1 before the fight. Against each other it's a little up in the air as they have to be good enough not to get smash by the other sides chaff but cheap enough to flood the battlefield with warm body's. Space Marines are their worst enemy because you bring 100k to fight 10 Space Marines, lose 20k to kill 1 and still have the other 9 to fight. Heroes on the other hand if you bring the same 100k and kill them by the time they got 80k kills you still win.
