Which of the six principles was most difficult to interpret? And why?
The Contiguity Principle gave some initial pause for thought. I think this is because I was trying to visualise it, as the principle in itself is pretty fundamental. The other five seemed intuitive enough and have become more familiar since this article was written back in 2013.
This article and accompanying research certainly illuminate the underlying reasons for my own longstanding aversion to Powerpoint and took me back to the ‘Death by Powerpoint’ video of about the same vintage. https://youtu.be/Iwpi1Lm6dFo
Which one principle might not work so well with L2 learners?
The Redundancy Principle could present issues for L2 learners as juggling between complementary versions of the same information would be considered a higher level skill. Although it might well also be argued that this could work wonders for paraphrasing skills.
I would also query the sweeping statement in the Modality Principle “that hearing verbal information is more beneficial than seeing the same information presented in text form on the screen”, whilst recognising that there are many L2 learners – especially outside of an EAP context where literacy skills are paramount – who may find written text largely incomprehensible and yet have excellent listening skills. As @robert points out, the assertion is not a given with L1 learners either. This leads to the wider point that squinting to read on a publicly shared, glaring and distant slideshow (as opposed to a close and personal sheet of paper or screen) is, by definition, never going to be an optimum reading experience for anyone in the first place.
As an overall design principle, any visual is really only valid when it serves to assist comprehension – rather than detracting and distracting. (Decorative considerations arguably aside.) How this is achieved will depend on a host of factors in addition to learning objectives and language level.