He is bigger, stronger and faster. He doesn't have the vision that Smith had and isn't on the same level abilitywise on being able to make defenders miss. Smith could see holes before they developed. He could find creases and wind his way through defenses. Defenders had trouble getting a bead on him.
Despite not being that fast, he had a lot of long runs because he understood how to slip tackles and take angles where it was difficult to catch him. And even though he wasn't the biggest player, he had great balance that allowed him to deliver a blow or drive a defender with great leverage.
The same arguments against Smith's ability have been made concerning pretty much all bigger, faster and stronger backs who have come along. His achievements are often just chalked up to longevity. Those who do that have a very limited understanding of what makes a back great.
Elliott could end up being one of the greatest of all time, but is he better than Smith right now? Is he better than Smith when he's not aware of where he is on the field and goes out of bounds on a TD catch his team needs? Is he better than Smith when he tries to hurdle a defender when the opportunity is there to possibly break into the open field for a much bigger run?
I'm glad we have Elliott, but he has a ways to go to become the type of player where you can say he was even as good as Smith.