I saw this on the askreddit "what books changed your perspective" thread. But in the comments, it said that a lot of historians had issues with it. I don't want to read anything that is not true/valid/credible. Thanks for the help.
Objectively speaking, Zinn does not do a very good job following the 'historical method' -- the standards and best practices commonly used by modern historians. Many (sorry, no source, but you can google it) believe he also had an ideological motive for writing the book, as opposed to a desire to write an objective, accurate work that goes deeper in certain topics or covers new ground.
The weaknesses in his narrative are generally in two areas:
Sources: History is filled with all kinds of sources, but the trick is discerning the ones that are the most complete, reliable, relevant, objective, error-free, etc. Zinn ignores many of the best primary sources from the periods of history and topics he addresses, and instead goes to sources that are far less reliable or are secondary instead of primary. He picks and chooses from the narrow set of sources that support the story he's interested in telling, or takes some material out of context, so his work ends up being more storytelling than 'history'.
Reasoning: For any given topic or period, historians work to discern from a source or multiple sources what actually happened in an objective sense. This can be difficult, sometimes requiring educated guessing, thinking in terms of odds, or framing something in the pattern of what we know about how similar things happened in similar contexts. Anyhow, Zinn's reasoning, analysis, interpretation, is generally driven again by the ideological story he has set out to tell instead of what is thought reasonable by most other historians.