Interacting VLE data (K values, activity coefficients, fugacities etc) is not currently stored or utilized in the flow calculations. For example, if you have a multi-component chemical mixture with more than one component changing phase at any point in the system FluidFlow makes an estimation of vapor phase composition based on ideality. Therefore the product may not be suitable for use where you have many components each changing phase as we flow through a system. Then again it actually may be, using simplifying assumptions, often each case should be individually considered when you have flows of complex mixtures. Primarily FluidFlow is a tool for designing and optimising flow systems rather than attempting to be a stand alone process simulator.
Also, we currently have no provision of catering for mass transfer between phases due to solubility of a gas for example. Similarly one could not make a mixture of say hydrogen chloride gas and water and expect to get the properties of hydrochloric acid.