Yes there is a limitation. The Congress only has the enumerated Power to make laws regarding promoting and providing for the general welfare, and that is the exact reason why the Constitution was formulated. Although Congress is the font of all laws by, of, and for the general population, it is not given the Power to execute nor of justifying its laws. Those Powers are obviously distributed exclusively to the other two branches of the government. Furthermore, none of those Powers are given to the fourth estate, but the Power to criticise and howsomever communicate to the general population is.
I think one of us is confused.
If Congress has the "enumerated Power to make laws regarding promoting and providing for the general welfare", then what limit would exist aside from whatever any particular Congressman might think is beneficial?
Are you suggesting that the Supreme Court need only concern itself with whether a law "promotes or provides for the general welfare" in order to decide whether such law is Constitutional? I don't think so.