One of the biggest differences is that Neoplatonism, at least in its early forms, does not personify the divine. We may identify God the Father with the Neoplatonic "One," but in doing so, we either have to attribute a persona to the abstract singularity that is "the One," or we have to deny the personified descriptions of God the Father as described in the Bible. Similarly, we can conceptualize Christ as an emanation of the One, but that is substantially different from what is taught in (small "o") orthodox Christianity.
Again, Augustine's doctrine of original sin may have some resonance with the Neoplatonist idea of the world as debased and distanced from its divine source, but the idea of seeing this in the actions of Adam is a departure. Ideas such as confession and substitutionary atonement would likewise be alien to the abstract and depersonalized metaphysics of Plotinus.
Perhaps the best way to conceptualize it is that Augustine and others used Neoplatonism as a lens through which to understand and interpret aspects of the Christian scriptures and traditions. If you have an abstract, universalist, and metaphorical understanding of Christian religious philosophy, you can still see it as aligned with Neoplatonism, but the more concrete, literal, and specific you get, the less the two philosophies are compatible.