(1) The universe is not at all like, nor does it resemble, human artifacts.
(2) You cannot argue from the parts of a thing to the whole of that very thing. An ashtray, a glove compartment, and a mirror are parts of a car but they do not resemnle a car. Human artifacts, it may be granted, are parts of the universe, but you cannot conclude from this fat that the universe itself is like a human artifact.
(3) The argument fails because in this instance an analogy is drawn between human artifacts and only one thing of its kind, i.e., the universe. We have no other instances to check the analogy against. If we had many instances of many universes before us and all of them resembled human artifacts, we would have some reason to think, perhaps, that universes were also designed. Since there is no other universe with which to compare this (our) universe, we cannot be sure it was designed or simply came into being on its own.
(4) Arguments from analogy are not without problems. In the absence of a shared principle for picking out those aspects of a thing and the thing or those things with which it is being compared, it is not clear why this or that aspect is selected for comparison. So, in the absence of a principled way for picking out the aspects to be compared, if it is claimed that the universe must have had a designer because it resembles human artifacts and human artifacts all had designers, then we might just as well conclude that the designer of the universe was like most designers of human artifacts, i.e., someone with a pair of hands, a pair of eyes and a nose.
(5) In the absence of a principled way of sorting out the relevant aspects in virtue of which the analogy is drawn, we might as well note since the universe is flawed and imperfect in any number of ways, its designer, i.e., G-d, must be flawed and imperfect.
(6) So, too, thought Hume, a comparison between the universe and a plant might just as easily be made as comparing the universe to (with) human artifacts. Since plants come from plants, we might then conclude from this analogy that G-d is a plant.
(7) Despite its intricacy and complexity, the universe might nonetheless have resulted from mere chance
(8) The final move of most arguments from design, i.e., that because the universe is grand, its designer must be grand, is too weak to count as much of a case for the existence of a Grand Designer.