JBC: There is no such thing as an objective person. If you say you disagree with durant because you do not like their bias that is one thing, but to label them as "bad historians" you should offer evidence of faulty research or deliberate misinformation.
Being a "bad historian" is not necessarily based on "faulty research" or "deliberate misinformation." In my opinion, Will Durant was very biased in his opinions in his history. For example, he was wasn't overly enamored with the Roman Catholic Church, especially in his book on Medieval History. Does one have to be enamored with the RCC? No, but one should lay out the history of the period to the best of his ability and let the reader come to his or her own conclusions. And different readers of the Durant's history series will undoubedly have various points of view as they likely would reading a more objective account of the period or any historical period.
Admiral Chester Nimitz, one of the great naval leaders of WWII was once asked by Professor E. B. Potter, a professional historian (and later biographer of Adm Nimitz) about a thorny issue that he intended to present in a book that he (Potter) was writing on the Naval War in the Pacific. A series of actions off the Philippine Islands led to the destruction of 3 of 5 jeep carriers (CVE--escort carriers) by a superior Japanese Battleship Task Force because of a failure in judgement by the famous Adm Bull Halsey. Furthermore, the amphibious landing forces off the Phillippines coast were threated by this powerful Japanese naval force. Prof. Potter did not want to demean the ilustrious naval career of an otherwise brilliant Admiral so he asked Adm. Nimitz how he should go about descirbing these series of actions in his book (a history book!). Adm Nimitz suggested that Prof. Potter lay the situation out as accurately and as objectively as he could and let the reader come to his own conclusions. Good advice in my opinion. No human being is totally objective, but the effort to be objective will likely bear good fruit.
It is also (or still) my opinion that the Durants did not practice this methodlogy in their history series. I'm not saying that they didn't put a lot of work into it--years of their lives in fact--and I'm not saying that there are gross errors in the facts that they report. It is clear that their efforts were monumental in scope, time taken, and results. I just opine that if you want to read a good history of any particular historical period you can do much better than the Durants. For example, for the Medieval Age, I would recommend the several books of Prof. Norman Cantor. They are eminently readable by the motiviated and reasonably intelligent layman who is not a specialist in the period.
I still stand by my statement that you can do better than reading the Durants' works.