Marines may not be Uber in one general area like some armies, but they;re downright great at a LOT of things.
Nids and Orcs are both great assault armies, but poor shooting armies. Tau and Guard are bot great shooting armies, but poor in assault. Eldar are good at both, with more focus on assault these days.
But man for man, Space Marines are in the top ranks for both shooting and assault (thogh chapter rules or traits could put you more in one camp than the other)
They can assault as good or better than anyone short of Nids, khorneites and certain eldar aspect warriors (though if you take sheer numbers into it, orcs and gaunts can be a real threat), and they can shoot as good or better than anyone short of Tau (though again, factoring in numbers, the Guard are up there)
you can do both at once, or you can specialize and be even better in one area while not becomming too weak in others.
Blood Angels, for example, are one of the best marine chapters for assault (I'd put Space wolves above us), but we;re not really any worse at shooting tnay a codex xhapter. Just a rare chance that a unit might move when you dont; want it to. Doesn;t stop us from fielding las/plas squads and the like though.