The fatal flaw of fundamentalism is its absolute reliance on its interpretation of words which are, in themselves, only translations (usually into archaic English) of what was originally written and spoken in the Hebrew and Greek languages, rarely taking into account the understandings of the ancients, calling them dead traditions.
They claim to be doctors of the Law but they understand neither the arguments they are using nor the opinions they are upholding.
Other forms of Christianity—Orthodoxy, Catholicism, classic Protestantism, even modern expressions not derived from the other three—certainly are also flawed, but not necessarily fatally.
What makes a fatal flaw? Anything which debars us from really hearing the words of Jesus and doing what He commands. We may say we know the Lord. We may say we believe solely in scripture. We may know and accept the four spiritual laws. We may even work miracles. But whose voice do we hear without letting our additions to it or our interpretations of it prevent us from observing it, prevent us from really following Him?
Start with the actual words of Jesus. Then let the apostles show us how to apply them. Do not start with the apostles, or worse yet, modern advocates of their own philosophies, and then add Jesus’ words as occasion requires. Start with Jesus. Join the apostles. Hear Him as they hear Him, and with them do what He does.