The major strength of Diamond's theory is that it seems to make a lot of sense. It seems to explain why European countries have, in general, been stronger than those from other continents. Diamond's theory is quite convincing. He does a good job of showing why Eurasia was better able to produce early civilizations and why those civilizations were able to grow and become strong.
The real weakness of Diamond's theory is that it cannot be proven or disproven. Aspects of Diamond's theory can be falsified. We can prove whether he is right or wrong about the number of domesticable plants in a certain area, for example. However, we can never actually prove that cultural or racial explanations have nothing to do with the dominance of European countries. We just have to take his theory (like most in the social sciences) on faith.