If you ask someone who finds the Enneagram types useful, you'll probably hear that they are accurate. However, there's a difference between being "accurate" and being "useful." Accuracy is about how well the types describe reality and in particular people's experience. Usefulness is about how well the types help me understand myself and others. There are many models and theories used in psychology and other disciplines that are not completely accurate but are useful. The Enneagram types are one such model.
The accuracy of the Enneagram types depends to a great deal on how well the user of the model can interpret the elements of the model. The difficulty in learning the Enneagram types doesn't come from a lack of learning resources. The difficulty comes from learning how to interpret all the concepts and labels used. For example, the passion of type 7 is said to be "gluttony." If I look up "gluttony" in the dictionary it says something like this:
habitual greed or excess in eating.
That is NOT what type 7 is about. The idea of "passion" and the word "gluttony" need to be reinterpreted in terms of the Enneagram types. Often that reinterpretation begins with a statement such as the passion of type 7 is "a gluttony of the mind" and then continues. There are dozens and dozens of words like "passion" and "gluttony" that are associated with the types. That's a lot of interpretation to learn.
The Enneagram symbol itself is yet another matter. What do the lines and arrows mean (i.e., stress/security points, directions of integration/disintegrations, law of 3, law of 7 or octaves)? What's the significance of the adjacent types (i.e., wing types)? Why is there a gap at the bottom between 5 and 4? What's with the triads (e.g., centers of intelligence 891, 234, 567; horneyvian triads 126, 459, 783)? What about other concepts not apparent on the symbol (e.g., instincts, Tritype, levels of development)?
The question to ask at this point isn't "how accurate all this is?" but "how good am I at interpreting and using it all?" and "is it worth an investment in my time to become good at interpreting it?"
There are a lot of resources out there on the Enneagram types but they are certainly not all equal. A complete training by a particular school will teach you how to use their interpretation of the model but may cost more than you want to spend. Learning it from the free sources on the Internet is a hit or miss approach. Some information will be useful but some will also be contradicting and confusing. The books can be more consistent and complete than the Internet resources but can also be contradicting and confusing. The challenge is weeding through what's good or not when you may not know enough to even do that. It helps to have a reliable source that can cut through much of the noise and get you on the right path to making sense of the types. Where do you find that? Well, I'm biased toward the book Enneagram User Guide which I hope to make available very soon.