As a Guard player, i disagree.
The Russ isn't supernaturally durable. It's just a giant hunk of munitorium clog iron. Giving it a 2+, the kind of roll reserved for terminator armour and ceramite (the slabshield is wierd, but it IS a ceramicte plate the size of a car and the thickness of a man' s arm), would kinda cheapen what a 2+ means for me.
And anyway it's already really tough and difficult to remove for its points. It doesn't ignore everything. It takes it right on the chin, and that's fine because it has ten buddies who can take it too. It wins on point efficiency, not toughness, just like any other Guard unit.
It's not the invulnerable superman. It's Rocky, having taken 10 shotgun-blast punches to the face, and somehow still standing.