These questions come up regularly when one works in the realm of multicolor profiling and printing. Unfortunately, accurate information is not always so easy to find. One area of frequent misunderstanding is how multicolor separations are calculated. Output profiles do not, as a rule, impose any limitation in the number of output channels but rather use as many channels as necessary to reach the lowest deltaE to the target. The simple reasoning that one shouldn't need more than 4 colors in a 7C set to match any in-gamut color is based on the assumption that we have ideal colorants (including a really dark black) that overprint perfectly. In CMYK this assumption leads to the idea that we need only two chromatic colors plus black to build any color. But we all know that in practice images frequently require all 4 colors--this is especially obvious in building dark colors--we just can't get them dark enough any other way, and if that dark color is also saturated this is even more true. It's no different in CMYK+N: If you have a very dark red, nearly black, the build may include CMYK plus red or orange, for example, as that may be the mathematically shortest route to get that color. I realize that certain systems attempt to get around this logic in order to conserve output channels, and plates, but this is usually when converting spot colors, not images. That path uses a different conversion with different algorithms.
There is also a widespread concern that printing with two many colors will lead to screen angle conflicts. This is overstated. If one is forced to use AM screening any issues can be minimized by use of sensible angle assignments: black and violet can print on top of each other--both very, very dark, and complementary colors such as cyan and orange or magenta and green can safely share the same angle. Where they do overprint it is in very dark colors where moiré or dot-on-dot effects don't cause a problem.
I have seen nothing wrong with i1Profiler's "math" in multicolor, but results can be seriously compromised by the MC profiling charts it generates: These simply do not have enough samples of medium and light colors while greatly oversampling dark overprints. The result is a smooth-looking profile that consists largely of interpolated values where real data should be present. There is scant relief in importing third-party charts, as i1P has a bug that causes the profile to have a total ink limit of 200%. That is a problem. I can't comment on Schnitzel's example as I haven't seen it and don't know what his criteria are for "ugly": Is this assessment based on measured or visual assessment or simply the appearance of unexpected colors in the separation? It may be both, but beyond specifying the ink limits and black generation there is nothing to be done about the number of channels used to convert an image unless one is willing to accept less accurate results.
