If there is one fundamental rule in 40K, it has to be thus: mobility rules. The ability to get your men to where they need to be faster than your opponent wins games more often than not. If your opponent has faster units than you do, you better find a way to slow them down or a way to speed your guys up, or you will find your army folding like a cheap metal chair.
Which brings me to Eldar grav tanks. I love their look, I love their speed, I love their weapons, I simply love fielding them. But god, do I hate trying to destroy one. Facing more than two is a nightmare for me. Despite the best efforts of metalguns, lascannons, krak missiles, they do not go down for me it seems. Or, when they do go down, all of them go down together. Drives me batty. Don't even get me started on Falcons and their "holofield". Arrrrrgggg!!!
Marines in particular suffer when Eldar grav tanks are doing their invincible bit because Rhinos are the opposite of Wave Serpents and falcons. Slower with less weaponry and armour. Add to the mix the sheer number of Str 6 or greater weapons at the disposal of Eldar and Rhinos are an endangered species on the battlefields of the 40th millennium. So for a marine player, when facing multiple grav tanks he has to move as fast as possible to take them down and if he fails typically he loses the game.