Well, moving on... Marx actually caught on to that little snag, and added the amendment that this went only for socially useful labor. Since it's somewhat difficult to quantify social usefulness, I'm gonna assume this means demand. Ergo, he's assuming that two articles with the same demand will be worth the same given the same labor time. This isn't so bad (though it's steering a little close to the neoclassicist theory of marginal utility -- one that most Marxists reject), but it's still not really up to snuff, because it ignores the importance of capital (as in physical capital) in production. If I make a chair by hand in two days by hand and you carve an identical chair in an hour then the two chairs are obviously worth the same. And even if mine is worth more (it likely is because it's handmade), it'd have to be worth some 20 times more, at least, for it to be worth the same, labor-time-wise, as yours (not likely).
This is a rather important point, because it essentially destroys the Marxist theory of labor exploitation -- which is the central point of Marx's entire economic model. Capital machinery obviously adds value to production (significantly so in fact), and acquisition of capital machinery is made possible by capital investment, which is the contribution of capitalists to production. Ergo, evidently capitalists add value to the production process above and beyond that produced by workers, and thus the value they extract from the production process is not a labor surplus.
Now, if you wanna argue that the value derived by capitalists is excessive, then that's fair but it falls in the realm of generic socialism, rather than Marx's "scientific" socialism. It's somewhat moot, given that nowadays the value derived by senior management far exceeds that derived by capitalists themselves, as do their powers; and senior managers are technically workers.