There have been many threads in the past detailing the variety of problems, you can find them in the FAQ.

The short version of historians' views of Diamond is that he operates under a set of problematic assumptions which are a matter of disciplinary training. He's fundamentally a scientist, and is thus looking to identify and describe universal laws the govern nature. In that broad sense, then, he looks for the universal. Historians, on the other hand, are deeply troubled by universals, because any deep investigation of the past always presents you with its specificity. Professional historians thus default not to the universal, but to the particular. So, for historians, Diamond's work is deeply problematic because he assumes that human societies can and should develop along particular teleological paths, elements that he assumes to be universal to human history but which historians view as the very particular results of historical contexts and events..

As for the originality of his research, it's really not original at all. The historian Alfred Crosby (most famous for the concept of the "Columbian Exchange") made basically the same arguments that Diamond makes in his book Ecological Imperialism, though Crosby published more than a decade before Diamond and handles the topic in a much more careful manner. Crosby understands the limitations of the argument, while Diamond does not, and so Diamond extends the argument to untenable conclusions.

You're much better off ditching Diamond for Crosby. His book is a classic, it's been through multiple editions, and I'm sure you could find it used online for under $5.

I know in the archaeology field he is considered a pop historian who sacrifices accuracy for readability. No one would reference his work in a serious piece of work unless they were pointing out the mistakes he made or the misconceptions he has.