1) impossible to get free from
2) impossible to disentangle or undo
3) hopelessly involved or complex
The interaction of flux and alloys/surface finishes is involved and complex. I think we've gotten to the point where we can take "hopeless" out of the description of flux though.
It is still widely thought that a flux is 'more powerful' or 'more active' than another. That's like saying "John is better than Joe" – John is better at what?!? Part of the confusion with fluxes came from the very chemical nature of the fluxes. Since certain chemicals react with certain oxides better over different temperature ranges, it would only be accurate to say that Flux A is better than Flux B for soldering alloy "X" to surface finish "Y" with "Z" profile. And even then you'd still be discounting the effects of Oxygen level during reflow, reflow equipment type, and other lesser variables.
When I began testing different fluxes to examine the effect surface finish had on solderability, I expected each flux to solder to copper differently. For example - lets call the wetting a function of that flux (f), and the difficulty of the surface a nominal value copper=1. It made sense to me that for a given alloy "A", the result would be 1A(f), soldering to nickel would be something like 0.5A(f). That's just not the way fluxes work though. Yes, it is more complicated than a simple formula, but flux activity is known through experience. Extensive testing has shown what works, and to what degree. There is no longer an excuse to guess at a preferred flux for ball-attach applications.