Morison isn't as good as a work like Tully and Parshall's "Shattered Sword" or Eric Lundstrom's "The First Team" and similar modern works that make extensive use of Japanese sources and allied sources (including Morison often as not). Morison had far more limited access to these sources and in particular the Japanese language secondary research that had been done after Morison published his official history.

Morison did have good access to U.S. Naval sources and he's generally reliable for his interpretation of these, except on a few occasions where the USN sources turned out themselves to be in error (i.e. Savo Island). Occasionally there are places where he is slightly political in trying to avoid a sharp critique of USN officers.

There is no getting around the fact that Morison shows some Racism in his writing, but he was writing in the 1940s and 50s when that would have been a common American point of view. There is no way to understand the Second World War without coming to terms with racist and nationalist attitudes shown by the people who participated in it, and Morison lacked the distance and time to separate his views from the men who he interviewed as sources.

In short, I would treat Morison's semi-official history as something like a distillation of primary sources from a certain point of view, despite his literary qualities, rather than a secondary source written by a modern historian. This is sort of how I'd treat other writers from the same period like S.L.A Marshall.