There is quite a lot of written material on the Imagon lens (on there are two articles and a brochure). One of the things written is that more direct lightning is prefered to soft lights. I.e. much "tougher" light than what is normal for portraiture.

Very true, although this becomes less important as the effective aperture is reduced and the image becomes, overall, sharper. The need for harder light might be an argument against an Imagon as a "first" portrait lens, particularly if one is planning to use electronic flash. Flash spotlights do exist, but they are less common, more expensive, and harder to improvise than softboxes.

Another issue is that because of scatter, the effective contrast of the film is lower when the image is very diffused, and this may need to be compensated in development.

For all that, they do make lovely portraits, if you happen to like the soft-focus genre and if it suits your subject.